One of the mostrepeated arguments in favor of a division 1A playoff is that the NCAA sponsors a playoff for every other division. The counterargument is that in Division 1A the entire season is one 14 weeklong playoff. We can see why it is right to have a playoff in all other divisions and why there's no need for one in D1A just by analyzing the schedule graphs.
Div  Teams  Games  CI%  Diam  APL 
3†  228  1092  14.44  10  4.33 
2  150  742  19.11  8  3.87 
FCS  124  590  24.80  6  3.18 
FBS  120  675  46.30  4  2.48 
^{†} D3 doesn't include the 10 NESCAA teams who only play each other and do not participate in the playoff.
 Teams
 is the number of teams in the (sub)division (except as noted for D3)
 Games
 is the number of games in which both teams are from the same (sub)division.
 CI%
 is the percentage of all team pairs in the (sub)division who either play each other or have at least one common opponent.
 Diam
 is the largest number of steps required to connect any team to every other team in the (sub)division with a "team plays B plays … plays Z plays otherteam" chain. This is the diameter of the games graph.
 APL
 is the Average Path Length for the (sub)division. Smaller values indicate greater connectivity.
All of the (sub)divisions except the FBS need a playoff because their regularseason schedules don't provide enough information to determine the relative order of teams. The only way to "connect" all the teams is to have teams that have no common opponents meet in a singleelimination tournament.
FCS, D2 and D3 just don't have the money for airplane trips and hotel rooms, so their regular seasons end with less than a quarter of the teampairs even having a common opponent. Their schedules result in weaklyconnected "regions" within which it is possible to find the top teams, but to find the best teams over the entire division, they have to arrange for the top teams from different regions to play each other.
Even if money were no object, with more teams and fewer regularseason games than D1, to define a regularseason as meaningful as the FBS portion of D1's would require centralized schedule management and a redistribution of teams across conferences. In other words, it's basically impossible for the other divisions to have a schedule "like" the FBS schedule and without that there isn't enough data to have a "mythical" champion based upon polls or computer rankings (though Wolfe, Massey, and a few others do a credible job for all divisions) so they get to crown real champions.
The first is my most common rant, and I don't care if you're tired of reading it, I'll repeat it until the problem goes away. Games against FCS teams eliminate FBSvsFBS scheduling opportunities, and once those were allowed to count towards bowleligibility every year the number of them became and stayed a bit too large. These games are necessary (the change from counting one every four years to every year was done to provide needed revenue opportunities for the FCS teams) but it would've been better to have gone to a "count it once every two years" instead of every year.
The second was the Pac 10's decision to use the permanent 12^{th} game to add a 9^{th} conference game. Commentators everywhere lauded the Pac 10 and called for other conferences to adopt a roundrobin conference schedule. But adding a conference game does nothing to improve the regularseason schedule (just moves one team from the Opponents' Opponents bucket to the Opponents bucket), and by taking 10 potential nonconference matchups out of the games graph they made it worse. When we had the first two seasons of 12game schedules in 2002 we had things like a home and home between USC and Auburn (a rare regular season matchup of the Pac 10 and SEC) but when the 12game season was made permanent they added the extra conference game instead.


The chart to the left shows just how different the postseason would be if there had been a 16team playoff in 2008. It also shows that every conference  especially the nonAQ conferences  would be lesswell represented in a 16team playoff than in the 68team bowl season.
The selection criteria might well not be the BCS rankings, but most proposals assume all conference champions would automatically qualify, so last year there would've been at least three teams outside of the top 16 by any standard.
Might there better alternatives? It's easy to see that even if we had a playoff, there'd still be about the same number of disgruntled fans.

The results for one such rating after conference championship weekend are shown at the right. Note that if we had had:
So, no matter how many teams are included in a putative playoff, there will be as many (more, actually) teams fans' with a defensible (if not necessarily legitimate) claim that their team was "unfairly" excluded. And this is the "best case" scenario where "the best x" teams are selected and seeded. Including conference champions automatically could only make the numbers worse.