Everyone was so used to having this unparalleled control of the engine. Your performance depended highly on your technical command of the game.

Then we hear we're getting a sequel, in HD, with new characters and stages. People got excited, started speculating about it being Melee 2.0, raised their hopes high and had them dashed by the first location tests.

A lot of people dislike Brawl because of the removal of Melee's advanced techniques. Sakurai is on record in places saying, in different words, that he dislikes advanced techniques because they create a rift between the casual and competitive players that can't be repaired once you cross it. The game was slow and floaty and had a forgiving buffer. It had weird ledge mechanics, way less hitstun, air dodge didn't put you in special fall. Behavior we had trained for almost a decade suddenly was bad play, because of the shield mechanics, and then we discover tripping, and it's like a slap in the face to the competitive scene that a lot of people took personally and just held a grudge ever since.

Those who didn't write Brawl off there start playing it competitively, where we discover shit like standing grab infinites and stalling techniques like planking and scrooging so degenerate we have to add a ledge grab limit and lower the time of matches by removing a stock.

Some people dislike it because their mains that were strong in Melee suddenly weren't high tier, or because their character changed undesirably and they had a hard time adapting. At the same time, Metaknight is so meta-warping we end up banning him for a while only to lift the ban so the Japanese would come play in US events. He is so influential your character's matchup with Metaknight almost single-handedly determines its tournament viability.

All of it piled up; most people who wanted a competitive sequel to Melee saw Brawl as a step in the wrong direction.

Most of the people who hate Brawl are probably bitter because they were expecting a HD embrace of the competitive scene, and they can't see past that grudge into the game that's actually there, but I'd wager the majority of the smash scene appreciates Brawl for what it is, though, another entry in the franchise.