>>6694662
For the context of when it came out, it makes a lot of sense. Super Mario Bros. was a legitimate phenomenon in Japan. People would play it for hours and hours even after they'd already beaten it just because of how much fun they had. There were also a bunch of secrets to find, warps, etc which kept people hunting after every secret had been found. The original game was pretty easy up until the final World, so all that time players spent gitting gud was mostly just exerted on speed running the game for the sheer fun of it. Then add in the Famicom Disk System, which Nintendo could use a killer app for, even if it's a rush job.
All this taken into consideration, Super Mario Bros. 2 was a totally logical step forward. Super Mario Bros. was a hit on Famicom, and with the extra power from the FDS, they could include Luigi as a playable character for some variety, allow players to save, and do stuff like wind effects on top of the existing game engine. The game was designed for those SMB pros in mind, you spent all that time mastering SMB just for the fun of it? Here's a game where you can truly prove yourself to be a master at the game. Plus, they used their knowledge of how people played SMB to do sneaky tricks like having warp pipes that take the player backwards, include Posion Mushrooms to show that looking for secrets is going to require the player be on their toes, etc. And for that audience, I'm sure it was nothing short of a success.