Most of the explanation lay in existing infrastructure and entire networks of professionals well established in New York City and Las Vegas. These cities have been promoting and hosting major combat sports (i.e., boxing) events for literally generations. Said professionals don't want to travel so far away to build up new networks, in a region with cities that are much more industrial and blue collar than what they're used to.
Two things to note:
First, Texas has plenty of combat sport athletes and fans, but demographics dictates a greater likelihood for an interest in boxing than mixed martial arts. Second, Texas has its own industries and sport events that its residents go nuts for.
Also, as an aside: anyone who has ever spent time in Dallas or Houston and thinks you can't find a sports bar or some other public watering hole to watch a mixed martial arts event hasn't left their hotel room. I mean, needing to drive an hour to find a place streaming an MMA? What coastal elitist bullshit.
