Searching is the main reason.
Say you want to find some specific information on handlebars, windshields or lighting. If you search the entire site you'll find every post with those words.
Having many categorized sub-forums allows a member to search a very specific area to find the most useful information available. Otherwise, it could be frustrating searching through common discussion posts to find the intended information.
Some have stated they believe that posting in one of the many sub-forums will not get the visibility of say posting in the general sub-forum, however, most members on the site do not view the side one sub-forum at a time, they use the 'New Posts' or 'Unread Posts' links. Those are search tools that list the new activity on the site in a chronological order,- newest posts first', regardless of the actual location of the post.