There were a number of actions that the Big XII took after Texas and the other three SWC schools merged with the Big 8 that were against Nebraska's wishes. Several 11-1 conference votes with Nebraska in the minority, with even the other Big 8 members siding with Texas and the Big XII offices being relocated to Texas. Cap that off with the Big XII's formation destroying the rivarly with Oklahoma, and there weren't a great number of reasons to stay. More stability, more money, and better academic affiliations were available with the move to the Big XII. While Kansas, Kansas State, and Iowa State were convenient travel locations in the Big XII, none of them were real rivals, except for maybe a very short period in football in the late 90s and early 2000s (the Callahan era ushered in the end of that budding rivalry).

It wasn't just Texas, although Texas certainly was the leader of the pack in the conference offices. If Texas were to become part of the Big 10+?, it seems highly unlikely that they would be able to exert the kind of influence over the midwestern schools, including some of the historical Big 10 powers, that they were able to claim in the Big XII. That said, I'm not exactly clamoring for Texas to be back in the same conference either.