The consensus on here seems to be that new Smash players don't hate Melee, they hate the players. Well, it's exactly the opposite for Melee players most of the time. Many Melee players hate the new Smash games with a burning passion, and that hatred isn't entirely unjustified. Since Melee it is no secret that Sakurai deliberately tried to undermine the competitive Smash community. He slowed the pace, removed hitstun and edge hogging, and added tripping to hurt the game's competitive potential on purpose. He looked at a community of players literally in love with the game the way it was, with all its tech skill and combos, and slapped them all in the face. It was utterly soul crushing. Only after seeing that people still wanted to play the game competitively did Sakurai let up and add a little hitstun back with Smash 4 and Ultimate. But his anti-competitive game design is still apparent throughout the new games.
If ultimate was literally Melee with updated graphics and new characters everyone would love it. Ultimate players are just stuck with what they have and don't know any better and don't like being told they are playing an inferior version of Smash that is much easier to play and worse competitively, even if it's true. The word "elitist" gets thrown around by Smash 4 and Ultimate players a lot, but that's kind of an odd criticism to use in a competitive gaming community. Distinguishing the best is kind of the entire point. The truth is that Melee, more than any other Smash, distinguishes the best from the average. You see ridiculous upsets from complete randoms in the newer smashes all the time, because the newer games are inherently less competitive. You would never see something like that in Melee. It's hard to point that out to someone who only plays Ultimate without threatening the sense of importance they attach to being good at it. So even if you approach an Ultimate player with respect you'll have a hard time just telling them the truth without annoying them, and most don't approach with respect because they feel utter disgust towards the newer games.