Well, because some silly people decided that one type of cartoon and one type of comic was better than the other, and whined whenever the other kind was talked about. So they separated them in an effort to please everyone. I think it was the people who'd later get /jp/ made for them that whined the most.
Which was silly.
They should have made an Animation board and a Picture Novel board instead, rather than Animu and Mango and Comics and Cartoons.