Vegas is a strange airport, they prefer that people hang out at the slot machines all over the airport (and lose money) than go to lounges (and do something other than gamble away their money). So it may be that Las Vegas airport discourages lounges?

Keep in mind, an airline can't build a lounge on its own. The airport has to agree to it.

Plus, it's one of the airports with the most leisure (and the least business) fliers. But lounges are more for business travelers. Once you factor that in, it probably drops way down the list.