Las Vegas is the pugilistic sports capital of the U.S. Not to mention the fact that the UFC organization itself is based in Las Vegas.
Who cares where the spectators are from? The point is they are in Las Vegas and are filling the seats.
What he said! ^^ Have you ever watched other combat sports like boxing? Vegas is a major draw and people come from ALL OVER THE WORLD to watch fights in Vegas. It's an exciting city with plenty of stuff to do before and after the fight with tons of Casinos, bars, extravagant clubs all crammed into an insane strip.
Why wouldn't they throw a lot of blockbuster cards there?