His offense. At this point, it's a well known, one dimensional passing offense. It was probably a well known one dimensional passing offense, by our 3rd Superbowl I think and it just never changed much.
Really. That's probably the biggest reason.
It works wonders against 90% of the NFL because he outsmarts them pre-snap and outexecutes them with his vet receivers. But it's simple, he's been running it for years, it's built on execution, well known by now, with lots of tape on it. Against smart defensive coaches with speedy, powerful, dominant defenses, with defenders who can man up, it just doesn't stand up anymore.
And of course, in Peyton's case, a lot of the money goes to his offense, him and his receivers. That doesn't help other areas of the team.
So when he gets to the playoffs, he runs into playoff defenses...and you have what happened so many times before and during the Superbowl.
Rinse and repeat.