When we have one vowel and one consonant and we want to add 'ing', we usually double the last consonant. Why don't we add an extra 'x' to the word 'fix'? We don't double 'w' and 'y' maybe because they are semi-vowels but what about 'x'?
The letter x is never doubled as in tax: taxing, taxed. Probably because x is not found double in other terms.
Words of one syllable ending in one vowel + one consonant, double the last letter before a vowel suffix.
We don't double up the final consonant when it's w, x or y.
4.1.2 Some consonant letters are never or almost never written double: h, j, q, v, w, x, y.
Almost all the exceptions occur in compound words, for example bathhouse, beachhead, fishhook, hitchhiker, witchhunt and withhold, where the first <h> is always part of a digraph or trigraph ending the first element of the compound word; also bowwow, glowworm, powwow, skew(-)whiff (usually spelt with the hyphen, however) and slowworm. There are also a few slang words with <vv>: bevvy, bovver, chivvy (also spelt chivy), civvy, divvy, flivver, luvv-y/ie, navvy, revving, savvy, skivvy, spivv(er)y. Some brandnames deliberately flout this rule, e.g. Exxon.
I think it helps to consider why we do double most other letters. To a first approximation, there are three major historical reasons for this:
- In Old English, double letters like /ss/ actually represented extra-long consonant sounds. (We no longer have such consonant sounds normally, but they can still happen at word boundaries: if you carefully enunciate "in nations" or "but tides", you'll notice that the "n n" and "t t" sound longer than the "nn" in "innate" or the "t" in "betide".) The doubled letters have survived even though the extra-long sounds did not.
- The extra-long consonant sounds tended to correspond to certain other pronunciation differences, some of which still exist (or at least, have evolved in such a way that they're still different). Most relevantly for your example of fixing, Old English had a tendency to use short vowels in a syllable that ended with a consonant and long vowels in a syllable that ended with the vowel; so hop and hopping (pronounced hop and hop- + -ping) would have a short o whereas hope and hoping (pronounced ho- + -pə and ho- + -ping) would have a long o. So the double letters have stuck around, and even been used in many newer words, because they help indicate a pronunciation difference that isn't otherwise indicated.
- Additionally, English has borrowed many words from other languages that used double letters to represent extra-long consonants (or that used double letters where there had once been extra-long consonants), and these have tended to reinforce the pattern.
None of these reasons has ever really applied to <x>; it's always been inherently extra-long, since it's pronounced /ks/ or /gz/, and it's always triggered the same sorts of tendencies as double letters. (For example, it's almost never preceded by a long vowel, though there are exceptions such as hoax and coax.) So while we could certainly imagine a parallel universe where the letter-doubling pattern got extended to <x> just by analogy, it's not too surprising that that hasn't happened in ours . . . yet.
That said, I've noticed that people who oppose vaccinations are often called anti-vaxxers for short, and who knows but that this may be the start of a trend. It may yet happen that <xx> becomes a normal part of English spelling, at which point perhaps people will start to write fixxing and sexxy and boxxed, and maybe even maxximum and buxxom!