Perhaps because the Latin ante Christum natum is longer. Literally "before Christ born".
Edit: And probably due to influence of the Church. Dates with A.D. were more frequent. The time before the birth of Christ was not so interesting for the men of the Church. And if they spoke of the creation of the world or of other events of the Old Testament they had no exact dates.
The Sydney [New South Wales] Morning Herald posted this letter to the editor under the title "Why do we use the Latin AD, but the English BC?" in its issue of May 7, 2005:
AD is an abbreviation of anno Domini Nostri Jesu Christi, Latin for "in the year of our Lord Jesus Christ". The era we now call BC used to be known as "a.C.n.", an abbreviation of "Ante Christum Natum", which is Latin for "before the birth of Christ".
Why the terminology changed from Latin to English is a matter of speculation. In non-English speaking countries, they tended to use the local language: in French, "avant J.C." (before Jesus Christ); in German, "v. Chr. Geb.", an abbreviation of "vor Christi Geburt" (before Christ's birth).
As with most things these days there is also a politically correct version of AD and BC. The years we know of as AD are now to be known as CE, "Common Era", and the years we know of as BC are to be known as BCE, "Before Common Era". How a mere change of abbreviation can be deemed politically correct, when the underlying concept of a time-system based on a minority religion remains the same, is anybody's guess!
Paul Dodd, Docklands, Victoria
It came in response to the question "Why do we use the Latin AD (anno Domini), but the English BC (before Christ). What do non-English speaking countries call BC?"
Instances of the wording 'before Christ'
A Google Books search finds instances of "[year] before Christ" from as early as John Higgins, The Mirour for Magistrates: Wherein may bee seene, by examples passed in this Realme with how greeuous plagues vices are punished in great Princes and Magistrates (1575/1587). Higgens consistently introduces his ancient historical examples with dates identified in the form "the yeare [or yeere] before Christ. [year]." For instance:
How Hvmber the king of Hunnes minding to conquer Britaine, was drowned in the arm of sea called Humber, about the yeere before Christ. 1085.
How King Locrinvs the eldest sonne of Brutus liued vitiously, and was slaine in battaile by his wyfe Queene Guendoline, the yeere before Christ. 1064.
How Queene Elstride the Concubine and second wife of king Locrinus was miserably drowned by Queene Guendoline, The yeare before Christ. 1064.
How the Lady Sabrine daughter of King Locrinus and Elstride, was drowned by Queene Guendoline, the yeare before Christ. 1064.
How King Madan for his euill life was slayne by Wolues, The yeare before Christ, 1009.
How King Malin was slayne by his brother King Mempricius, the yeare before Christ, 1009.
How King Mempricius geuen to all lust was deuoured by wolues, the yeare before Christ, 989.
How Qveene Cordila in dispaire slew her selfe, The yeare before Christ, 800.
How King Morgan of Albany was slayne at Glamorgan, in Wales, The yeare before Christ, 766.
How King Iago Dyed of the Lethargy, about the yeare before Christ, 612.
...and so on.
Similarly, from William Camden, Remaines Concerning Brittaine: But Especially England, and the Inhabitants Thereof, fourth impression (1629):
The Greekes referre this inuention [anagrams] to Licophron (as Isaac Tzetzes hath in his Preface to his obscure Poeme Cassandra) who was one of those Poets which the Greekes called The seuen Starres, or Pleiades, and flourished about the yeare 38o, before Christ, in the time of Ptolomaus Philadelphus King of Ægypt, whose name he thus Anagramatised.
'Anno Domini' and 'Ante Christum [Natum]' in otherwise English-language texts
A Google Books search for anno domini yields matches in English-language texts from as early as 1577. From Edward Hellowes's 1577 translation of Antonio de Guevara, The Familiar Epistles of Sir Anthonie of Gueuara, Preacher, Chronicler, and Counseller to the Emerour Charles the Fifth (1577):
As concerning the rest that your fatherhoode insite and incommend unto me, frier Benet your subiect and my friend, may say hat I did speake therein unto his Maiestie, and what he answered me, which presently was dispatched. No more, but that the grace Dei nostri Iesu Christi sit tecum & mecum. From Madrid, ye twelfth day of March, Anno domini. 1522.
On two later occasions in this book, the translator renders specific years by the shorter form "Anno 1527" and "Anno 1541."
In contrast, the terms ante Christum natum (a. Ch. n.) and ante Christum (a. C.) rarely appear in English-language texts, and date back only to in Google Books search results. C.N. Casper, A Vocabulary of Terms, Phrases and Abbreviations in English, German, French, Italian, Dutch, Latin, etc. employed in Literature, the Graphic Arts and the Book, Stationery and Printing Trades, etc. (1889) includes the following entries:
a. C., Ante Christum (La.), Before Christ.
a. Chr. n., ante Christum natum (La.), before the birth of Christ.
And Lutheran Quarterly (1951) [combined snippets] matches "ante Christum natum" against "post Christum natum":
With the coming of Christ into the life of man a new aeon has begun. The era ante Christum natum and post Christum natum indicate that in Christ's incarnation human history has a new beginning. But these two eras are not rent asunder, as though the one did not have any relation to the other.
But the earliest English-language text to use ante Christum natum casually in the midst of vernacular English that a Google Books search finds is Pierre Eggermont, The Chronology of the Reign of Asoka Moriya: A Comparison of the Data of the Asoka Inscriptions and the Data of the Tradition (1956) in, for example, this chapter title:
FIFTH CHAPTER: THE CONVERSION OF THE BUDDHA-ERA INTO THE ERA ANTE CHRISTUM NATUM AND THE ESTABLISHMENT OF THE YEARS OF REIGN OF ASOKA IN THE ERA ANTE CHRISTUM NATUM
The shortened form ante Christum does appear in some English-language books dating to the eighteenth century. In particular, Arthur Bedford, The Scripture Chronology Demonstrated by Astronomical Calculations (1730) includes many instances worded along these lines:
Table VII, or Calculation II. The Calculation of the Moon's ϕασις for the Longitude and Latitude of Cyparisson, (being the place where the Ark was built) in the first Year and first Month, when the Flood happen'd, Anno ante Christum 2352. ...
And Thomas Hogg, The Fabulous History of the Ancient Kingdom of Cornwall (1827) seems entirely at ease with the wording:
Julius Cæsar made his first appearance in front of the cliffs at Dover, on the 23d of August, 55 years, ante Christum, according to the Calendar now in use ; and after three o'clock, in the afternoon of that day, sailed with the tide eight miles, before he landed.
Likewise, Ezra Pound resorts to it multiple times in Cantos LII–LXXI (1940) in this recurrent style:
Yeou taught men to break branches
Seu Gin set up the stage and taught barter,
taught the knotting of cords
Fou Hi taught men to grow barley
2837 ante Christum
and they know still where his tomb is
by the high cypress between the strong walls.
Sorting out the abbreviations
Webster's Academic Dictionary: A Dictionary of the English Language ... Abridged from Webster's International Dictionary (1895) includes in its list of "Abbreviations Used in Writing and Printing" these relevant abbreviations:
A. A. C. (Anno ante Christum.) In the year before Christ.
A. C. (Ante Christum.) Before Christ; Archchancellor.
A. D. (Anno Domini.) In the year of our Lord; Archduke.
B. C. Before Christ; British Columbia.
Several noteworthy points are evident in this selection—and in the items that are absent from this list. Most obvious at the outset is the stack-up of the abbreviations A.A.C., A.C., and A.D. Under these circumstances, using A.C. as an abbreviation for "After Christ" is not a realistic option. But there is no objective reason why the dictionary couldn't have included A.P.C. (for Anno Post Christum) and P.C. (for Post Christum) as forms meaning "after the birth of Christ." Their omission thus suggests a lack of support in English for a more logical alternative to Anno Domini or "Year of Our Lord" to use as a complement to "Anno Ante Christum" or "Ante Christum" or "Before Christ." Y.O.L. for "Year of Our Lord" seems never to have caught on, but the full phrase was quite common for several centuries.
The Catholic Encyclopedia (1907) has listings for A.C. ("Ante Christum—Before Christ"), A.C.N. ("Ante Christum Natum—Before the Birth of Christ"), and A.D. ("Anno Domini—Year of Our Lord"), but nothing for P.C. (Post Christum) or P.C.N. (Post Christum Natum). Here, again, the failure to counterbalance A.C. and A.C.N. with P.C. and P.C.N. strongly suggests that the sequence of abbreviations started with A.D. and only later addressed the question of what to call the years before A.D. In settling on A.C. and A.C.N., the Church in effect declined to express the earlier times in direct opposition to Anno Domini—as, for example, with Anno Ante Dominum. Perhaps Church leaders felt that the A.A.D. option put too starkly the absence of the Lord from the world.
In any event, The Catholic Encyclopedia describes a prevailing division of time into A.C. (Ante Christum) or B.C. (Before Christ) on the one hand and A.D. (Anno Domini) on the other. Neither AC nor BC explicitly represents the word birth in its abbreviation, as ACN (Ante Christum Natum) or BBC (Before the Birth of Christ) would have; so perhaps it is misleading to think of BC and AD as representing, respectively, "Before Christ" and "After Christ"; AD seems more readily to accommodate the notion of Christ's era as ongoing—almost as "During Christ"—whereas the truncated PC (Post Christum) suggests an end to the event of Christ's birth and a subsequent era disconnected from Christ.
In 1798, Stephen Jones, Sheridan Improved: A General Pronouncing and Explanatory Dictionary of the English Language, third edition, excludes both A. C. and B. C. from its list titled "Explanation of Common Abbreviations or Contractions of Words"—although it does include listings for A. D. ("Anno Domini, in the year of our Lord") and A. M. ("Anno Mundi, in the year of the world"). Noah Webster, An American Dictionary of the English Language (1828) doesn't list any abbreviations for the names of the eras before and after the birth of Christ.
However, B.C. does appear as an abbreviation given equal weight with A.C. in William Savage, A Dictionary of the Art of Printing (1841):
A. C. — Ante Christum. Before the Birth of Christ.
B. C. — Before Christ.
A. D. — Anno Domini. In the Year of our Lord.
An. A. C. — Anno ante Christum. In the Year before Christ.
However, the relative unease with dates before Christ as against dates in the Christian era can be seen in this passage from Richard Grey, *https://books.google.com/books?id=GCUAAAAAQAAJ&printsec=frontcover&dq=%22Memoria+Technica%22&hl=en&sa=X&ved=0ahUKEwjD4_echvzOAhUH_WMKHaAiB5gQ6AEIIjAB#v=onepage&q=%22Inachus%22&f=false), fourth edition (1756):
Thus King John began his Reign A.D. 199, (one Thousand being understood to be added, as I shall shew hereafter;) but this may be expres'd by anou, or boun, or ann, I make Choice of the last, for then 'tis but calling him Jann instead of John, and you have the time almost in his Name. Thus, Inachus, King of Argos, began his Reign in the Year before Christ 1856; with а very small Variation in the Spelling, 'tis his Name Inakus.
The earliest use of B.C. I could find in an admittedly slapdash search of Google Books is from James Anderson & John Entick, The Constitutions of the Ancient and Honourable Fraternity of Free and Accepted Masons (1767):
MARCELLUS shed Tears for him [Archimedes] as a public Loss to the Learned, and gave him an honourable Burial in the Year of Rome S37 —— A. M. 3792 | B. C. 212 while Hannibal distressed Italy.
In this excerpt, A. M. refers to Anno Mundi (Year of the World), counted from its origin.
A second early instance appears in Joseph Priestly, A Description of a New Chart of History (1786):
It [the Roman conquest of Britain] was begun in the year 43, under the emperor Claudius, and was completed in 78 under Domitian. Julius Cæsar had invaded this island in the year 54 and 53 B. C. but he made no conquest, his forces being repulsed by the natives.
The issue of how to date events that occurred prior to the birth of Christ arose in English by 1587—and may well have occupied English speakers from a much earlier date. The first recorded efforts that I turned up used the sensible wording "The yeere before Christ. XXXX." But abbreviated forms of "before Christ" (or of the Latin "ante Christum") seem not to have become widespread until the late 1700s at earliest.
Meanwhile, "Anno Domini" (A.D.) and "Year of our Lord" (no abbreviated form) were standard forms in English from a very early date. Because A.D. seems to have come into English long before either A.C. or B.C. became popular, any conflicts in abbreviation form or underlying phrases were likely to be resolved in its favor. This is indeed what happened.
There was never a serious possibility that A.D. would be jettisoned in favor of the logical complement of "before Christ" or "ante Christum" (namely, "after Christ" or "post Christum"), as those complements never had any weight in English writing as an alternative to "Anno Domini" or "year of our Lord." But on the other hand, there was no popular counterpart to "Anno Domini/year of our Lord" for the period before Christ's birth. The result was that English adopted a not-very-logical pairing of a "year of" expression and a "before/after" expression.
The remaining issue—how B.C. (Before Christ) triumphed over A.C. (Ante Christum)—remains unexplained, but it seems possible that writers deemed B.C. a less ambiguous abbreviation than A.C., given that "before Christ" and "after Christ" were common vernacular ways of referring to the pre–Anno Domini era.