The reason is TV ratings. OSU/TCU/TT/BU would increase the TV ratings average of the Pac, thus increasing the value of a potential new TV contract.
The other great thing about adding 4 schools to the Pac, for Pac schools and their fans, is the return to the original Pac 8. Ship the desert and mountain schools to the east division with the former Big 12 teams and voila, the original Pac 8 is back baby! Which is great for nostalgia but also to get annual games between USC and Oregon, and Washington. Those are big money games that the Pac currently leaves on the table 50% of the time.
Adding 4 more teams to the Pac actually increases the # of games between original Pac schools.