Expansion is voluntary. That is why it is not happening.
Bottom line is that the Big 12 teams would make less money per team while not gaining much with expansion. That is why it will not happen. BYU has an inflated opinion of their program. PAC 12 did not even want them.
BYU is not in the Pac12 due to the conference not wishing to kowtow to BYU's demands, like they didn't kowtow to UT's demands of an independent TV network.
It's because the ten members of the Big12 Conference are satisfied with the current arrangement that expansion won't happen soon. As soon as something bad, or "unfair" happens due to being a ten team conference, the Big12 will look to expand.