Nintendo was still new to the sequel rituals when they released Super Mario Bros 2 on the NES, and boy did they go away from the norm for what a sequel was supposed to be. Being almost a complete departure from the original NES title, in Japan the title used for the US SMB2 was a completely different title, what was there in the North American version barely said this was a Mario title. Barely.
Gone were the basics of Mario lore- no jumping on enemies to kill them, now jumping on the enemy just gave Mario, or one of the other three characters, a ride along the path. No jumping from underneath to hit coin blocks to earn money, no more castles to traverse and then we get hit with the biggest screw job when you beat it, bigger than the Montreal Screw Job in WWE
Was Super Mario Bros 2 all bad though? Nope, far from it, it was just a major departure from what fans were expecting, still fans bought it and word spread. The game was still fun, gamers enjoyed the negative world aspect (find a potion and throw it down and enter the door), the different enemies and the adventuring aspects that were missing in the first game (entering caves and buildings within the level and completing tasks and then leaving).
All in all, Super Mario Bros 2 was just different enough to cause fans to either love it or hate it. Me personally, I loved it. The imaginative ways the bosses were handled was a welcome departure from the way Bowser was easily dispatched in the first title.