They're just not that good. The Saints, Colts and Vikings are. So in order for the Patriots to win, they have to make some special things happen and those teams have to be a little bit off.
This is where, I think, Belichick is at his best. Like Parcells, he doesn't get caught up in romanticizing the past when assessing his team. "You are what you are," is the phrase that's appropriate. Parcells used to say that you couldn't claim a team was "better than their record shows. If you're 5-6 or 11-1 or 10-6, that's what you are. No better, no worse."
So Bill will take a very harsh, cold look at the good and bad and try to use what he has in the best way it can be used. No trying to mold this or that to what he wants. You play the cards you're dealt in the way they can best be played.
And if they don't win a Super Bowl, that doesn't mean Bill isn't at the top of his game or that the Patriots have lost some mystique. It just means they're not that good this year.