As soon as a championship is won, the players put on commemorative hats and the team starts selling championship merchandise. In order to have that merchandise on hand, both teams must make it in advance, which means a whole bunch of merchandise for the losing team very quickly becomes worthless. The clothing is shipped off to the impoverished.
Of course, something like a "Patriots 19-0" t-shirt isn't worthless at all to a NY Giants fan. Aaron Kaplowitz journeyed to a remote village in Nicaragua, and managed to buy some merchandise from incredulous inhabitants. Here's the story.