Because his "skill set" that makes him so good in boxing, wouldn't do shit in MMA. MMA unlike boxing isn't one dimentional, and his awesome skill set of running around the ring ducking punches isn't enough.
I think it's been proven over and over, that if you had to be one dimentional but very good at something, it's much better to come from wrestling then boxing. If Lesnar was a giant 280 Lb boxer, he wouldn't be as successful.
khoveraki covered most of it, but also...
Boxing isn't a style designed to control your opponent like wrestling.
If someone is outboxing a boxer, what does a boxer do?
If someone is outboxing a wrestler, the wrestler can take him down. Or if a wrestler fears the sub game of a BJJ player he can use his wrestling in reverse to keep the fight standing. Being able to control where the fight goes is a huge advantage. Another reason why wrestlers find a lot of success is the work ethic it takes to become an elite wrestler is unbelievable. Also they have years of experience in cutting weight.