Why don't you separate the novels into categories, but the short stories you do? It kinda doesn't make sense.
They were originally separated into categories but Daniel recently edited the site and that was one of the changes made. I think reason as to why can be found here.
Also, if you create five sub-sections within 'Novels' you'll find a couple people post their stories in the Horror section, while another one posts in Drama. You'd have to go back and forth between sections the that are already pretty lean on content. Also, sections such as Comedy wouldn't receive many posts. It's just more convenient to stick them all together.
The *main* reason the novel section doesn't have subforums is simply that there isn't as much demand for genre devision as there is for short stories.