There are only two conferences left west of the plains--the Pac 12 and the MWC. With the projected growth of the west and the limitations on scholarships, there are simply going to be way too many quality athletes for the PAC to maintain complete hegemony. The quality of the MWC recruits will improve every year relative the pac and the other conferences. If BYU were to join, it would drastically increase the reputation of the conference and speed up the process. If I'm the Pac, I want BYU in indy because I want the MWC to be as weak as possible as long as possible.
Nor does joining the MWC necessarily hurt our chances of joining a bigger conference. Every school that has been invited by another conference has been able to join--regardless of affiliation. I don't buy that Indy makes it easier.