Some possible reasons, just off the top of my head:
- He doesn't like to dance.
- He doesn't like to go to parties.
- He hates to wear a tuxedo.
- He can't afford all the obligatory hullabaloo.
- None of his other friends are going.
- He doesn't understand what all the fuss is about.
Seriously, I never did understand what all the fuss that girls make over prom is for. Not until I was well out of college and had a fiancee to explain it to me, anyways. For women, prom seems to be a chance to be a princess for a day; to dress and act like completely mature adults with someone you like or love; a chance to dress up and fix up and makeup to be as beautiful as they've always wanted to be; a dress rehearsal, of sorts, for the wedding they've always imagined they would have. It's dream fulfillment, start to finish, and they encourage each other to make an even bigger deal out of it by trying to upstage each other.
But guys, especially teenage guys, just don't care about all that stuff. Sure they want to upstage each other, but that's about they only thing they have in common with the opposite sex at that age. They're more interested in having fun with each other, playing games, watching movies, buying their first cars, and so forth. If it's not about sports, it's about cars, and if it's not about cars, it's about computers or some other tangible toy. Fun and sex are priorities in their minds, but romance is not.
"Why doesn't he ask me to prom?" The answer is usually this simple: he doesn't particularly want to go, and you've never told him why you do.