The Mafia didn't make their money on hotels...
They didn't even really make it on gambling.
They made their money loaning money to gambling addicts and charging interest. Besides laundrying money that's why they got into casinos, a steady stream of desperate idiots who would take out loans.
The cheapness of Vegas was the same as a crack dealer giving out the first hit for free, just to try it.
A certain percentage get hooked immediately