My theories are:
-Perhaps Hilda's outfit was too revealing? I don't know about you, but it's possible that the writers of the Pokemon anime didn't want to start any controversy, so they ruled her out . I mean, they could have just changed her design to fit the anime. Then again, this theory's probably false because Misty wears short-shorts in the anime too and few people seemed to have complained about it.
-Maybe they wanted to make the anime an homage to the Kanto series? Like the others said, I think the writers had planned on making the Black and White anime a bit closer to home, you know, like the first season of the Pokemon anime where Ash's two main companions were both gym leaders at some point. After all, they did want to add that same kind of effect to the games, so why not for the anime as well?
-My third theory is that maybe they didn't really know what to do with her? It sounds silly, but Hilda is certainly not the only in-game character that has not made an appearance in the anime. Unova didn't really seem to have stuff like contests and what not; the kind of things that Ash's past female companions were into. I personally think that Hilda could have worked well as some sort of rival or mentor to Ash; that could've worked. I guess that the writers either lacked ideas on what to do with her, or just didn't want to add her in because she didn't seem to fit in the mood of the anime.
I think I'll have to go with the second theory, since it's the theory that makes the most amount of sense to me. That, and the anime writers just happen to not use some of the in-game Pokemon characters for whatever reason.