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.
It makes sense why champion like Merab mostly fights in Vegas even if not always for title defense because there are three big reasons which you already mentioned first is lower taxes for fighters in Nevada and second is Nevada Athletic Commission makes it easier to get approval for fights and third is Vegas is best place for entertainment which means more fans come leading to higher earnings from ticket and pay per view sales for champions.