I think that part of the reason for this is that the side-cast is often made to be more flamboyant, secretive, witty, etc... because the developers only have a short period of time in the story to get their personalities across, where-as the regular cast has a lot of story to develop, so they aren't as "flashy". I imagine that the side-cast people would not be as "cool" if they had a full story-line to themselves, as their story would end up with slow points, odd plot twists and the like.