Really, there's not even any discernable reason for those games to draw huge audiences. They're attractive games and popular franchises, sure, but none of them had can't-miss storylines or any of the four or five most popular players in the league.
So what's this mean? Just the NFL's audience, which is already mind-blowingly huge, is still growing. Even in today's fragmented pop culture atmosphere, with the number of entertainment options growing exponentially, the NFL manages to consume more and more of us. The list of the most popular things in the world goes something like:
3. The NFL
Also, it means that next year's inevitable work stoppage will be murdering one of the world's most valuable television packages. Even the loss of these ratings and the mountains of money they generate won't deter the owners and players from whining and bitching their way into depriving us all of sweet, magical football.