As a former resident of Vegas let me also throw this in, it is a city solely devoted to hosting people who don't live there. When the go to Toronto people have to book hotels 40 miles outside of the city just to find a place to stay, in Vegas you can stay inside the damn hotel it will be in, if it is too expensive then you stay 1/4 mile down the road for cheaper.
As further evidence of this look at the BR show that had to be moved because the UN was going to be there as well and take up the accommodations, that just doesn't happen in Vegas.
It also helps that there is other entertainment as well making it a great venue for hosting. Everyone loves to go see a UFC fight, but what to do before/after the fight. While some places have clubs, etc, most don't have the level and amount of entertainment that a playland like Vegas does (for over 21). Sure other places may pack more in attendance, but lets see them keep that up for 10+ shows a year.