The "Championship Belt" concept comes from boxing/wresting and is the idea that every season, the previous years champion (here the Super Bowl) starts with the belt, and each time they lose they pass the belt to the winning team who becomes the new sole owner.

The concept is explained here:

https://grantland.com/the-triangle/introducing-the-nba-regular-season-championship-belt/

(Text credited to @StopPunting )