There are different forces at work, here.
As much as I would love an anime forum (just look at my avatar), while I would definitly appreciate and post to it, I must admit that it won't see much traffic.
Tilted Knowledge has trouble getting enough traffic to warrant its existence and I can only imagine there's more interest in that than anime.
However, it's true that there are several very small forums already on TFP, the smallest interest groups being
Tilted Electronics and
Tilted Travelling. It's tempting to claim that
Tilted Programming is among the small ones, as well, but the reality is that that forum is brand new and has gotten a respectable amount of traffic, considering. The point is that if these small (tiny, I might say!) forums can exist then why can't Tilted Anime?
Of course, Tilted Anime will also encompass anime scores/soundtracks as well as manga. Throw in animation in general and it just might see enough traffic to warrant itself.
I might want to point out, in case it wasn't clear, that there are plenty of forums even smaller than the smallest
Interests forums I mentioned but they're special. Their existence is justified beyond their popularity, like
Coming Together or
Tilted Suggestions...