The first Super Bowl, Marv didn't have a curfew on the team and the team partied all week long and didn't take the Giants seriously after they destroyed the Raiders in the AFC Championship game and the Giants were playing with a backup QB. Also, neither the Bills coaches or players were smart enough to change their game plan when Belichek was dropping extra defenders in coverage and essentially giving the Bills the run.
The second Super Bowl, you can thank Coach Dickerson for running his mouth and insulting the Hogs, plus the d-bag from Harry Connick Jr's band for moving Thurman's helmet (Thurman would have scored a long TD on the 2nd play of the game).
The third Super Bowl, the Bills were outmatched, plus it didn't help that Levy only gave Reich very limited snaps as Kelly got reinjured and Reich came in and was not prepared.
The fourth Super Bowl, Bills had the lead at halftime, but it could have been more if Kelly wouldn't have tried a terrible shovel pass (which Jimmy Johnson saw during Super Bowl week), plus once Thurman fumbled and Dallas recovered it and ran it back for a TD, the Bills lost their momentum and couldn't recover.