Strength of schedule and distribution of talent fluctuates too much...it'd become too much like the NBA where the same teams always make the playoffs
Plus it'd wreak havoc with the scheduling format itself; say you did away with the 4-team divisions and just had two 16-team superconferences, it could be years for an AFC team like the Raiders would face an NFC team like the Niners (that is unless league completely realigns geographically into eastern and western conferences)
Best and quickest fix would be to expand the playoffs to include more wildcard teams, but then that opens up the scheduling conflict of how many if the top seeds get a bye week and are there an even number if matchups to provide the right number if winning teams to face the corresponding number of topseeded teams that got a bye? Football isn't like the MLB where you can have a quick turnaround between a one game playoff and a wild card game a couple days later; and it wouldn't be fair if one team got an extra long bye just so the wildcard matchups could even out.
[ Edited by Dr_Bill_Walsh on Dec 2, 2013 at 4:33 PM ]