As was the response in the similar thread about The Rock changing his character; if it ain't broke, don't fix it.
Cena's look is iconic, perhaps more so than that of Austin, HHH and Rock and closer to that of Flair and Hogan. Austin, HHH and Rock all rocked the black trunks with each adding a little something different to the look to stand out, but guys before and after them have the same attire with similar differentiating attributes.
Jorts though, are instantly identifiable with Cena. Sure, Indy guys wear the weird PVC looking versions but lets not try and compare Indy wrestler Y with wrestlings biggest full-time star. Cena is a brawling type who doesn't lie to himself or those he faces, and his self belief is both inspirational to himself and, I imagine, his fans. The jorts perhaps look a bit out of place and cause ridicule. Ridicule, that he overcomes. It's a subtle thing, but it's a relevant addition to his character and is a sum of the total package.
Plus, he went for the camouflaged design not too long ago, lest we forget.