In a recent press conference in Japan, Shigeru Miyamoto had this to say about the Mario Bros universe:

Every time I make a series, I want to make a completely new game. Especially when the younger you are as a director, you want to make something completely different. When we made Mario 1, 2, and 3, we always started from zero.

At first I was confused by that comment. Super Mario Bros 2 started from zero? No way, I thought, Miyamoto-san didn't even make Super Mario Bros 2, it was just a hacked version of Doki Doki Panic for the Famicom Disk System in Japan.

But then I remembered why SMB2 was just Doki Doki Panic with new characters. Miyamoto-san thought that the actual SMB2 would be too hard for US audiences, thus the switch. And since the press conference was in Japan, I can only assume he's talking about the Japanese version of SMB2 (later released on the Super NES in Mario All-Stars as "Super Mario Bros: The Lost Levels"). The american version of SMB2 was then later released in Japan as Super Mario Bros USA.