It probably wasn’t?
Is it worth more financially to be in the SEC than to be perpetually winning conference titles and ranked in the top five/ten all year every year? I understand lesser brand schools (Mizzou, Maryland, even A&M) switching conferences for the reasons they did more than the historic brands like UT, OU, USC, UCLA, etc.
(Although the Pac 12 was a disaster, so fair enough there.)
The Big 12 that UT and OU left was absolutely a solid conference that they controlled. Seemed bizarre to leave.