Ideally, conferences would be 8 or 10 teams, and confined to a coherent geographical region, like much of the conferences were before 1984. Things started to change with Oklahoma v NCAA, which allowed the individual schools the negotiate their own media rights, instead of the NCAA holding the rights of all the member institutions.
This meant that in order to get better deals with the broadcasters, the conferences had to create a more attractive offering of games and schools, while also trying to expand the number of potential viewers(customers) within their conference footprint.
Basically all of conference realignment going back 40 years ties back to that case: the SWC breaking up, the Big East being picked apart, and the raiding of the Big 12 and now OUT.