One reason is the media partners would probably rather work with a more flexible and capitalistic and non-snobby BigXII than with the antithesis of that in the PAC10/12. Imagine the increases power that Stanford/Cal will have in the leadership vacuum created by the exit of USC/UCLA.
There’s no growth potential for the PAC10/12 if they’d never be willing to add religious schools like BYU/Baylor nor “stoop” to adding academic lightweight schools like Boise st.
Geography also favors the BigXII being the leader in the merger too as there’s more growth potential in any time zone being feasible if/when the ACC starts to get poached.
Some PCA12 schools might also like the idea of owning their own Tier 3 rights. That’s never been an option in their current situation and represents more $$ for the schools joining the BigXII. So the AAV numbers beteeen the two conferences isn’t necessarily an apples to apples comparison.
For these reasons mainly, it’s in the greater good for the BigXII to raid/destroy the PAC12 rather than vice-a-versa.