Merab won his title in Vegas, but never defended it in Vegas, however, indeed he had a lot of fights there. I think there are three reasons for that. Taxes. I havent googled it, but I suppose fighters pay decreased income tax compared to other states. Licenses. Nevada athletic commission, Vegas is in Nevada, its easier to get allowance to perform. And entertainment. No other place is as entertaining as Vegas. In UFC, if it is a title fight, then a champ gets % out of ticket sales, PPV, everything. People would gladly go to Vegas than to other state with similar venue size.