I'm gonna go with the cardio thing. Th TUF HW season was unbearable to watch at times. No one wants to see two gassed out fat guys stumble around a ring or laying on each other. It doesn't make for a good fight and certainly doesn't draw in new fans who are channel surfing.
Does that mean all the lighter weight class fights are exciting? No, but to the average viewer it at least looks more like you are watching two athletes in a semi professional fight, instead of a freak show with two fat guys struggling to breath.