People are waiting for the Wii U version. It's 4-player, has HD graphics, and looks like more of an upgrade.
After a lot of people already bought SM 3D Land less than a year ago, they're not going to buy another 2 Mario games this year. So I think a lot of people are picking one or the other, which is the risk Nintendo always ran having two new Mario games within 4-5 months of each other.
That said, for a "B-Team" game developed in 9 months or whatever, NSMB2 is pretty damn good. I knew going in the graphics/music weren't going to be much different, but I think honestly this is the best NSMB game so far. The graphics are quite nice on a 3DS XL (3D snow + ash particles are awesome, not enough of that). The level design is terrific. Maybe I'm just off but I'm liking it a touch more than SM 3D Land even. I'm happy with my purchase, even though yes, I wish Nintendo had hired some extra music staff for new music (c'mon Ninty), and a few other artists to maybe create new level graphics (rain levels on the 3D screen would be awesome).
They need to add some DLC to give people who get the million coins something significant, but otherwise I'm cool with this game.
The real problem with Nintendo is they don't trust their other core IP anymore and they haven't bothered to develop any other character based IP. So once the Brain Training/Wii Fit/etc. crazes died down a bit, basically Nintendo has been banking on Mario to do all the heavy lifting.
Need to get 3DS sales back on track? Mario 3D Land and Mario Kart 7.
Need to launch the Wii U, must have a bankable title? NSMB U.
Uh oh, 3DS sales in the West are poor and we need to launch something big with the 3DS XL. Quick lets get NSMB2 out.