The reason people still play Melee is because Brawl was intentionally designed to be as anti-competitive as possible with random tripping, and the total removal of nearly all of the advanced movement and spacing options that sped up gameplay and gave a lot of characters the tools they needed to be able to throw hands with the likes of Sheik and Marth.
The absence of L-cancelling, wavedashing, double jump cancelling, hitstun, dash dancing, etc. killed the combo game and made Brawl a campfest, and nobody had the tools to be able to effectively fight against Metaknight, resulting in a stale metagame.
Smash 4 is still a good game, but even that entry still plays nothing like Melee. Almost all of the tech unique to Melee can't be found in ANY other fighting game out there, and people are still discovering new tech over 15 years later. At least with Street Fighter, there are other games that play similarly (Blazblue, Guilty Gear, etc.) even when new releases are announced.
Just because a game is new doesn't mean that everyone's obligated to drop everything and only play that shiniest, newest, game. There hasn't been a deeper Smash game since Melee, although 4 is still good.