First reason is the cap. How wonderful that the NFL set up sharing the wealth (of champion players) with a cap that prevents the winning SB team (especially) from keeping their great players.
Second-Free agency, another NFL "sShare the wealth" of players with incompetent teams that cannot draft if they stepped in front of an open window during a hurricane. More of the "spread the wealth mentality."
Third, not a bad one as it creates promotions, is the hiring away of the winning teams' coaches again because other teams are incompetent in their hirings.
WHo of us hasn't scratched our heads in amazement at some of the bone-headed hirings of other teams FO's? It's along list (JJ-Redskins-Raiders, Jets, etc)
Last, maybe most important, the carrot is gone, the rabbit's been caught, motivation wanes, and SOME players ease up living off their SB win and then because of thast get hurt and out for the season