There was a great article years ago, believe DesNews. Interviewed several associated with BYU athletics, some one the record some remained annoymous. Anyway, essentially PAC realized in 90's it wanted to expand and ADs knew BYU filled their stadiums and increased viewership. Asked for BYU. End result was that 2 universities said no because BYU's conservative religious views could never be accepted there.
Also, the U got in because Texas, OK, and others changed their mind after Colorado had already been invited and with egg on it's face the PAC needed a partner and the U was the last resort.