Orly? And they specifically told you that they weren't interested in NSMB?
I just asked 3 colleagues of mine, two of them are mums, and one mum said she and her daughter play mobile games, and she hasn't bought NSMBW because she thought it may be too hard for her daughter (using buttons seems complicated) but is very interested and considering getting it for her. The other says that her daughters love NSMBW, and they also play mobile games. The other is a Male, no kids, and said he wouldn't mind playing NSMB, but he wouldn't buy a U to play it (he mentioned the U himself).
What would you say is the single biggest and most obvious reason why NSMBU didn't match the sales pace of NSMBW?
Clearly the pricepoint and lack of other games to buy with it. That's paired with the fact that cheaper alternatives exist.