Many say that it's because it would make the season too long for the players...However, I believe it's about money. For years, the BCS has had multiple bowl games which have different sponsors. Having a playoff would eliminate some of these sponsors and therefore would make less money for the BCS. However, this doesn't have to be the case. The BCS could sell naming rights to the rounds of the BCS playoff at a higher cost (because it's more games) than it could for a single bowl game.
If it's about making the season too long for the players why do they wait 3/4 weeks between Conference championships and bowl games? I sure know it's not exams. Let's be honest, these guys don't go to class and they sure as hell don't take exams.
BCS, do these kids a justice and make a playoff.