### Why spell it _connoisseur_?

You’ve basically answered your own question here.

The French word has been spelt _connaître_ **for close to two centuries**.

_Connoisseur_ was borrowed into the English language some time around **three centuries** ago, when it was spelt that way in French.

The fact that French has changed the spelling of the _French_ since does not mean that English (who is, generally speaking, very conservative about changing spellings) will change the spelling of the _English_ word.

Similarly, around the same time as _connoisseur_ was borrowed, the verb itself was also borrowed as _reconnoitre_ (now spelt _reconnoiter_ in American English—one of the places where English has changed spellings, though in a more or less systematic fashion). That word is pronounced with the stress on the ‹oi›, which is pronounced as /oi/. ‘Updating’ the spelling to the current French spelling would make the spelling _less_ representative of the pronunciation of the word, which would be silly. When English spelling does get updated, at least it’s usually to make it _more_ representative of pronunciation.

French has undergone a sound change, whereby the older diphthong /oi/ split into two different forms: one was /wa/, the other /ai/, which was later monophthongised into /ɛ/. The first of these is still spelt ‹oi› (as in _François_), whereas the latter is now spelt ‹ai› (as in _Français_). But this change mainly happened at a stage where many words had already entered the English language, and thus it was not reflected in those English words.

Some words were borrowed later on, though, and in those words, the English pronunciation/spelling match that of Modern French: in _je ne sais quoi_, the last word is pronounced /kwa/ as in Modern French; and in _reconnaissance_ (which was borrowed about a century later than _reconnoitre_ and _connoisseur_), the spelling in French had already changed, so the English form matches the Modern French form.

### Why use the word at all?

Your second question seems to be doing the same as your first question: implicitly assuming that French does not change. This is of course not so.

If you look in a French dictionary, the word _connaisseur_ is indeed there. My Hachette _Dictionnaire de la Langue Française_, for example, defines it very simply:

> **connaisseur, euse** _a, n_ Expert en une chose.

The fact that this is not a word French people tend to use nowadays doesn’t mean they didn’t 300 years ago when it was borrowed into English. The fact that it was borrowed at all suggests that they did, in fact.

Whether a word stops being used and fades into obscurity or maintains its popularity in any given language is completely unpredictable. The fact that this word has all but vanished from colloquial French, but managed to stay alive and kicking in English, is quite random and could not have been predicted; but once the word was there in English, there’s no reason the English should stop using it just because the French did.