Well, the UFC is a business first and foremost and one that's attempting to break into the mainstream sports market. It's still difficult to sell the UFC to a lot of people because many people view it violent and barbaric, and that'll turn a lot of networks and buyers away. When people watch guys like Nick Diaz have a pre-pubescent meltdown at the weigh ins or flip off his opponents in the cage, they're not going to get a good image and going to think that MMA is just fighting when it's really smething more. Not to mention post fight brawls and regular use of narcotics don't bode well either.
Not to mention retiring after being bewildered by the safety first style of the Natural Born Killer!