The reason 2-D Mario will always outsell 3-D Mario is that 2-D games are more hardcore AND more casual at the same time.
2-D platforming gameplay is on the one hand, so simple anybody can do it (you have a jump button, you move side to side, that's it), but on the other hand allows for much greater precision and timing and difficulty in jumps. There are jumps in 8-1 of Super Mario Bros. 1 that are harder than any jump in any 3-D platforming game. The 3-D environment lets you jump in any direction, which means they have to make platforms gigantic for you to land on. 2-D games have pinpoint jumps that are insane.












