Moving to the SEC means more money. Even if you're an SEC bottom feeder, you're making more than you would as a contender in the Big 12.
This is the same logic people use to hope Nebraska will come back to the Big12 - an easier path to playoff relevancy. But the bigger argument (to the decision makers at Nebraska at least) is the extra $25M they get each year by being in the Big 10.
Suffice it to say, I don't think Texas or Oklahoma regret their choice