How did individual mechanics in Melee contribute to this? Once again, curious.
I think the Smash Bros series's strength over other fighting games is that combos aren't the backbone of the design and if you can pull them off it's a sign of skill. I'm guessing Brawl tipped the balance too much for the tastes of the competitive kind.
Everything in Melee added very fluid, deep, and complex mechanics while also being simple to understand. For example, hit-stun, Lcanceling, no auto-mated edge guarding, the air dodge system (allowing superior movement through Wave Dashing), character weight actually meant something, pointless in Brawl because of it's floatiness. Proper dash mechanics, e.g, being allowed to shield while in your initial dash animation, opening up more complex game play.
Hit-stun: Everything in Brawl just feels out of place. Common sense dictates if you hit someone in Melee, you're capable of following through due to your superior situation. Not true for Brawl. You read someones play, punish their play, then all of a sudden you get smacked with a move while your still in animation from the move you punished the player with. It just doesn't reward the superior player as well in Brawl as it does in Melee. And right/wrong decisions are less transparent because the game engine feels so bad.
Lcanceling: In the context of SSB64/Melee, it added fluid aerial combat. Something that absolutely does not exist in Brawl. In Melee you can pick any character and perform well if you're the better player. In Brawl the games physics imprison you. You're forced to pick characters that have zero landing lag on aerials because there is literally no other option. Lcanceling also added a mechanical depth towards the game due to it being something you needed to master, something that separated truly competitive players (people willing to practice for their achievements) from the people just wanting watered down game play.
Auto-mated edge guard: Is terrible and should not have been added in Brawl. It completely neuters properly edge guarding. If you outplay someone, get them off the stage, and then have an auto game play mechanic built into the games physics save them, that's terrible. You should be rewarded or punished for your decisions, not have them hampered by game play mechanics built to allow casuals to perform better.
Air Dodge: While there is nothing inherently wrong with Brawls air dodge, in addition to the games low hit stun and the removal of Wave Dashing all it did was hurt game play. Wave Dashing made characters useful. Imagine if Luigi in Brawl had a form of mobility like WDing. He would actually be viable. Removing mechanics just to remove them is a terrible game design philosophy. Competitive players love options. WDing was just another combat option.
Character weight: Weight meant something in Melee regarding death timers and combos, it was very important. Brawl is just floaty chunkiness that feels bad, that has no direction other than allowing newer players to feel better about not being experienced players.
Dashing/Shielding/Jumping: In Brawl all of these feels choppy. Can't shield from your dash animation, why? It makes no sense to remove something that helped you in combat. It was simply a decision made to water down game play so the transparency between good and bad players wasn't so high. Something a competitive game should NEVER do.
Game play felt fluid. The advanced techniques in the game were something that needed to be practiced extensively to be performed efficiently (something that's good for deep game play). In Brawl everything is watered down and handed to you on a silver platter, so less experienced players felt good about themselves without needing to dedicate the time towards actually practicing. These are just general explanations that could be explained much more deeply but would take way too long, especially considering Brawl players never listen anyways.