The main reason we don't have a playoff system is because athletics administrators, university presidents and conference commissioners are on the payroll and VIP list of bowl executives. They're basically being bribed to keep the bowls active. That's really about it.
A more minor issue is that if a playoff were to come into existence (which it appears it now will), they want to avoid giving an automatic bid to the smaller conferences, but to give automatic bids to specific conferences while leaving the others out would trigger possible antitrust action. It's not necessarily that they want to avoid giving money to those conferences, as they already do with the BCS, but they enjoy sucking on the power teet and would like to put distance between the haves and have nots.
Personally I think an 8-team playoff is the sweet spot. Not big enough that it diminishes the regular season too much, but just big enough to be entertaining and incorporate enough of college football. However, they'll go to this 'plus-one' model in a seeded variety (a 4-team playoff, in other words) just because it continues to incorporate bowls.