Hypothesis: Super Mario World and and 3D Land may be there to fill in a missing "tier" of growing up with Mario and learning to appreciate him.

[attaboy]

2D Mario=the best tech of the time
3D Mario=the evolution of Mario
NSMB=throwback to a style gamers wanted
3DLand/3DWorld=2D Mario w/ 3DS showcase

That's how I see it. 2D Mario is there to make money from old farts like me and little kids. 3D Mario is there to showcase the power of the hardware.