Because above all they are greedy and want as much cash as possible, don't want to split the pie anymore. Only if forced by some external factors for a CCG or otherwise would they expand and then likely east for wvu and tv market partners. Looks like the Big12 will likely be able to have a CCG with 10 so that's what they will do first unless the other conferences try and stop that, which it doesn't appear will happen.
I could see the ACC being raided by several conferences, including the Big12 only if they feel 2 teams and markets make them more money. There are rumbling that FSU isn't happy and the could try and make a move to the SEC. That or the Big10 adding an ACC team or two could start the next wave of expansion.
I don't see how the ACC getting raided helps byu at all because it just gives the Big10, Sec, and Big12 more potential east coast partners.
the PAC12 will never likely add more teams unless Tx teams were interested. Other western teams and markets in the MW don't move their money meter. They also don't want a Mormon school and have Sunday play in most sports except football.
In other words, byu is likely doomed, well probably.