Because:I don't see how we can't have a sub-forum in the Arts and Entertainment section dedicated to Anime/Manga.
It would be easier for new members to find a place to talk about their favourite series, instead of being worried that they're posting in the wrong section or that there are members that don't like it. The anime thread already has a lot of discussions going on at once.
It can't hurt to try, can it?
1) There are already forums perfectly available for this (nobody's going to fret about where to put a thread about Bleach). Branch out of the Anime thread; you don't need a subforum to do that.
2) It needlessly spreads out the threads. The Arts and Entertainment sections already don't get a whole lot of action and what little threads are active will be even more spread out.
3) The list of threads is already fairly long for the number of regular members.
4) The Arts and Entertainment forums are based on Mediums (TV, Books, Music, Movies), and not styles and genres (or countries of origin). This means that it breaks the standard convention (which makes it poor design) and that every thread in Anime would be easily placeable it it's appropriate medium.