Senator McCain is the dickeater that tried to ban the sport completely. At first there was no set rules and regulations so they could have technically had it anywhere. Once McCain opened his mouth practically every state started to ban it. Then states like Nevada and New Jersey stepped up and started hosting events. But the State Athletic Commission had set rules to which the UFC had to conform too.
Slowly but shortly more and more states have lifted their ban on the sport. Now Nevada, New Jersey, California, Ohio, Hawaii, New Mexico, Florida, Oregon, Iowa, Colorado, Texas, Connecticut, Louisiana, Oklahoma, Indiana, and Washington all allow MMA events in their state.
Missouri allows amatuer MMA events but pro is still banned.