Salary cap, the 3-4 defense and domes.
The salary cap ended the dominations of the larger market teams, which were mostly in the NFC, no longer to deeper pockets mean deeper talent.
The AFC has begun re-adopting the 3-4 defense which, if you've got the personel to pull it off, is a superior base defense which allows for a lot more creativity in pass rushing.
And the NFC has become more and more dome happy, while the AFC remains with just 2 dome teams. Come January dome teams suffer when forced to play outside.
I see it as more of a cyclic thing. The AFC dominated in the 70's, then the NFC owned the 80's and on up until the mid 90's. Now, the AFC is on top again. In another few years, the pendulum will swing back to the NFC.
If anything, I'd say it has more to do with the coaches and GM's finding the players who fit be it smash mouth, or West Coast offense. Gibbs, Parcells, and Walsh all became head coaches within about 4 years of each other, and got players who could play their designated role in the system, which coincided with the rise of the NFC. Chuck Noll and Don Shula did the same thing, as does Belichick and Andy Reid.
Well thought out theory.