In Super Mario Bros. 2, the gameplay mechanics were vastly different from the other 2D side-scrolling Mario games. For example, jumping on enemies didn’t kill them; instead, you picked them up. Is there any specific reason that Nintendo decided to take this route with the game, instead of sticking with the gameplay that Super Mario Bros. established?
I wouldn’t be surprised if this gets closed, but I’ll do my best to answer anyway as I happen to know the answer.
The original Super Mario Bros. 2 that was released in Japan is known as The Lost Levels in America and wasn’t released until 1993 as part of Super Mario All Stars (a great game in and of itself). When Nintendo of America got a hold of the Japanese version of Super Mario Bros. 2 they found the game to be too hard. According to Takashi Tezuka and Shigeru Miyamoto, this was actually the goal of the game, however NoA cancled the stateside release as a result.
Since The Lost Levels wasn’t to be released in America and Super Mario Brothers 3 was about to be released in Japan, Nintendo of America singled out a different game to take the place of Super Mario Brothers 2. That game was Yume Kōjō: Doki Doki Panic. Since it featured 4 different protagonists with varying stats, Toadstool and Toad were added to Mario and Luigi. Its also worth noting that this is the first game to feature Luigi as being taller than Mario because the character he was based on (Mama) was noticable taller than the character Mario was based on (Brother). The result is what is commonly known as Super Mario Brothers 2.