One reason is that the differences between American and
English are more involved than simply changing the spelling of a few
words. Punctuation and grammar are also involved. If you changed
behavior to behaviour in an otherwise AE sentence, the sentence would
then be wrong in both languages.
I can't seem to find the page now, but I seem to remember a policy page
where we've basically settled on a compromise, partially-invented
"international English" punctuation style for clarity and because it's
not really worth fighting over. The compromise included the
British-style "put punctuation _after_ closing quotation marks", and
something from US style that I can't remember. As for being "wrong",
that's only the case if you happen to be a [[en:prescription and
description|grammatical prescriptivist]], which not all of us are.
But as far as the spelling issue goes, it seems like a solution in
search of a problem. The current approach seems to be working well enough.