SMG's environment is a lot more controlled and less filled up.
It's kind of like when they did a game like God of War on the PS2, and the priority was graphics. So everything was very scripted and controlled to have a lot of processing power being used all the time on visuals. SMG also had a high priority on graphics, and was more of a controlled experience as well(not to the same degree as GOW, but I hope that's a good example).
NSMBW puts more emphasis on gameplay. While in SMG the processing power is used more fully to create the visuals, in NSMBW that capability is left for possibility in gameplay. For example, you may be playing single player and not doing everything that can happen on the screen which is a fraction of what the game can do, that wouldn't quite happen so often in SMG so the processing power could be used for visuals. So it's really a matter of priority coupled with the gameplay style. Also I think NSMBW looks fantastic.
You can't really use the SMG engine in NSMBW and expect better results while having the same gameplay style.







