As several people have said, the sub-forums are generally created based on demand. When a forum has a noticeable quantity of active threads on a subject (like talking about politics, or talking about sports); then we split it off into a sub-forum as it should have the users to keep the forum active, as well as help the people who want to talk about that topic see more of what's relevant.
Having a ton of slow or empty forums just makes the site look dead, and dilutes the conversation in the more general forums.
With that being said, now go make some threads about ball handling or pointing at guards.