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.
I don't think the pendulum is swinging back to the NFC any time soon, too many dome teams. Of course they could get so many that the dome stops being a problem in there. There certainly are a lot of inept GMs in the NFC these days.