The main reason is actually more on the practical side. It was exclusively released for the GameCube and hasn't gotten any remakes or rereleases, so new people don't have access to it unless they buy a GameCube and an expensive second-hand copy. It's all about bringing a classic to a new audience.
But having better graphics and more content would be amazing for the fans, too.
RE: [Old Stream Archive - 08-31-17] Paper Mario Pro Mode #8