My gut is that it's as simple as Vegas being viewed as a cow town, with UNLV not all that attractive, until recently. Then there was such an influx of pro sports that it in some ways cut against UNLV's value proposition, even as Vegas grew into an attractive market. Especially in the fall and winter, when the two major revenue sports are playing, you have hockey, football, and likely soon will have the NBA there as well.
I don't see why the mob would care about UNLV being in a legit conference as opposed to the MWC. More competition for dollars that isn't offset by getting more heads in beds? Maybe, I guess, but you would have a lot more of that with all the pro sports heading that way. Higher-profile contests coming to town would lead to more action in the casinos and other stuff.
Now it's a growing, major city with UNLV ... still not all that attractive. But two things on that front: we can bet Kliavkoff knows more about Vegas and what's going on there, with the mob and otherwise, than just about anyone. So if the Pac-12 isn't even interested, there's almost certainly a very good reason formulated by someone who has as good of data as you could get. Otherwise, though, my sense is that the Pac-12 passes and then then Big 12 grabs them shortly thereafter. Again, they're in the Pacific time zone but not all that far from Big 12 country.