While half the country lives in the "East," you only have so many time slots for "acceptable" starting times in ET -- those have almost all been taken. By moving west, the Big 12 becomes more flexible and can have late games -- say games starting at 8 or 9 ET -- too late for games located in the East to start -- but that people on the East coast will still watch (no travel time after the game, can have kids in bed, etc.)