Why? Because the potential for NO Big Ten or Pac 12 teams in the playoff reduces college football to a regional sport, and that substantially cuts the ratings for the playoff and championship games.
Well, the CFP committee is tasked with choosing the four best teams for the playoffs, not the the four best teams in terms of viewership for ESPN. The PAC 12 and BIG at least for this season are inferior to the ACC, BIG 12 and SEC.