Capitalism is just the free market. Whatever consumers want to buy is in demand, so someone produces it in order to supply that demand. So either the majority of hip-hop fans wanted the dumbed down nonsense, or there was/is a concerted effort by people in power to make sure the dumbed down nonsense remains in demand.
It's not hard to imagine the establishment being concerned about perhaps the most powerful and influential art form in the world being dominated by a pro-black/pro-youth/anti-establishment message, but without some kind of solid evidence to prove that, what caused the dumbing down of rap music will essentially be the same as the chicken or the egg debate.