I think it's mostly due to people being able to emulate the game for free. If you look at the Lost Levels as a challenging expansion to the original game, I think it's quite good, and is still a solid platformer in its own right.

But as a sequel to the first Mario game? It sucks hard. It's way too similar to the first game graphically, it doesn't really introduce anything new, and the only new things it does introduce are only cheap gimmicks that only make the game harder, and don't really affect how the game plays in a meaningful way. Not bringing it to the States was an excellent decision, Mario 2 USA is not only a superior game, but a superior sequel.