The only reason this Mario might have been more received then NSMBWU that I can think of is that 3D Land had massive first week sales when released and did very well. Thus, a much larger and grander follow-up to a game that had a lot of demand should expect big numbers also and move more units. I do think that 3D Land helped push more systems in Japan but as everyone notes, that's because handhelds are the systems that seem to be what sell there.
Although, I've said it before it and I'll say it again. As far as I'm concerned, the DS was the best system of the last generation. If I had to choose between a Wii, Xbox 360, PS3, DS, or PSP, DS would be an easy choice for me. Likewise, with the way the current generation is going, rigt now the 3DS would be my number 1 choice for the system to own but it is of course quite early in this generation. It could just be the Japanese are smart and realize that this is the case and that is why the DS and now 3DS are doing so well there.