Because PM isn't "basically Melee with more characters and just better functionality in general". It's more like a sequel to Melee, a new game that builds on its predecessor but with many key differences. Many of the new characters have shaped the meta in a different direction, it's not always as balls-out aggro as Melee is known for, and there's a lot of janky matchups to learn. Stages change a lot too, and create some polar counterpicks. And there are various tech differences, light shields are gone but now we have footstools, DACUS, and glide tosses to think about.
Some people will welcome the new changes, but others prefer Melee the way it is. And there's nothing wrong with that, it's all a matter of opinion. It's not going to be for everyone.
As for the gods, all of them have played PM and they win a lot just from their raw fundamentals, but I've seen them get caught up on matchup unfamiliarity sometimes.