Melee had an amazing Adventure mode because they made it short, but incredibly high quality and really well detailed. Made it replayable over and over again because of how great it was.
Then with Brawl Sakurai decided to turn Adventure mode into this like 5 hour story mode where you go through a platforming game fighting monsters and the level design was absolute crap, gameplay was a bore, story was stupid (not that I expected much though), and it was just bland overall. It was hyped up like crazy too and I was so let down by it.
So naturally people hated the Brawl Adventure Mode because how bad it was, so Sakurai just took this as "Oh people don't want an adventure mode" instead of "People like a short but well done adventure mode".