UNLV definitely has the potential to be good down the road, and I've said as much before.
But right now they're an abysmal team with no fanbase and no investment in football. Bringing them into the Big 12 right now would only result in them continually being obliterated by other quality teams and failing to build up any excitement or fanbase (hard to get into rooting for a team that considers a 37-10 loss a "pretty good" result).
In a different scenario, I see their upside. But the Big 12 can't wait on what might develop a decade down the road