This is a tournament being run by an outside organization. That organization owns the TV rights to the tournament. They can choose to (or not to) make games available for broadcast. Remember the Creighton game in Des Moines a few years ago? Same thing.
Iowa State can't just show up with a production crew and say "We want to show these games."
UNLV probably wasn't planning on playing in the non-televised game anyways.
The late decision to allow UNLV to stream the game would have come from the tournament organizer, and knowing who that is I wouldn't be surprised if UNLV had to sweeten the deal in order to make it happen. Plus, UNLV's website is through CBS. So that may have come into play as well.