More than anything, the Big12 is a powerhouse football conference, and over the past say 5 years or so has been a powerhouse basketball conference (outside of Kansas, who is always good). TV revenues are presumably huge, even without a dedicated network (that I am aware of...there isn't a Big12 Network, is there?) The Pac12 has done an absolutely horrific job with its TV contracts and has too many "deadweight" schools dragging down the conference rankings.
In the end, UCLA and USC bolted because the Pac12 was not getting the job done in terms of $$$ or in terms of competition for two schools that would aspire to win a football championship one day. Perhaps if the Pac had its crap together they could have stopped the bleeding from that, but as it stood the Pac "leadership" was nonexistent and the Big12 had solid opportunities to expand because of some defections to the SEC and that was basically that. Too small to compete with the big boys, too big to not get looted when it all hits the fan. The Pac is now sitting on 4 members, the MWC on 11 full members. It seems pretty likely to me that a merger is going to happen there. Maybe the Pac even has enough clout to get the new entity to be called the PacMount instead of the MountPac. But RIP Pac, it was nice knowing you.