It's called direct marketing. Really big money is being able to pack a stadium. When it is held at a stadium merchandise, beer, parking and the like are all big for the promoter. But along with that big venue comes the big risk. It's less risk for a big fight just being a big fight rather that a mega draw when the venue is sponsored by a casino. It'd more prudent for a promoter to take the sure bet and hard work out of promoting by taking the casino's bid and make the fight in Vegas. When the fight is held in Vegas people buy stuff around the fight and the casinos make the extras.

So if you have the biggest fight of the century (Pacquiao vs Mayweather Jr.) and it's a sure thing as a promoter where would you want to make the fight?