Conferences are in business to maximize profits for their teams. Tv revenue is the main part of that. For instance the SEC Network charges $1.30 /month for instate and $.25 for out of state. Adding A&M and the almost 8 million households in Texas increases their profits $8 mil a month just from 3rd tier rights. That doesn’t include 1st & 2nd tier that will also be renegotiated. Adding another team from Texas doesn’t help that number.
As for adding Texas to the SEC, it won’t happen. There is a block of schools that don’t want multiple teams from their state and will vote together to keep them out. Their best bet is the Big 10, but they will have to drop their network.
Houston’s problem is that they are in Texas and don’t add much TV money. If the Big 12 does expand, they will go out of state and add more TV’s.