He did was his coach told him to do. If it were Dungy they would have gone for it, but Fox is the most conservative coach in the league.
Don't pretend for a second that if Petyon WANTED to go for it that Fox would have said no. I remember Peyton on many a fourth down with the Colts, shooing away the FG team so he could go for it. Make no mistake, the offense in Denver IS Peyton Manning. He brought it from Indy. The key you are missing is that Peyton said he was in agreement with it. I think the cold and his arm got to him at that point. It was clear in OT that he had nothing left on the ball.