Same reason Marino doesn't have a ring. The team around him hasn't been that great. They can get by in the regular season by playing and beating bad, average and even good teams. But when you get into the playoffs and you get to the divisional round or AFCCG then you are playing a really good team. The Bills weaknesses (mostly on defense) get exposed. Allen hasn't been able to make up for that by himself up to this point. If Leonhard can build a strong defense, then you will see the Bills chances of winning a Super Bowl improve by a lot.