College sports are national and need to involve colleges from across the country or interest will wane.
Not a perfect analogy, but what if the NBA moved all their small market teams to the NYC area and LA areas. You could probably pencil it--all the adversising, money, people. But in the long run, a league with five New York teams and five LA teams is lame. The product would be less cool, it would become less national, etc.
Again--they need to see how great March Madness is. I know basketball and football are different, but there are elements that are the same too. The more that football playoffs are viewed as a legit national playoff (and not regional/VIP), the better.