Clearly the pricepoint and lack of other games to buy with it. That's paired with the fact that cheaper alternatives exist.
That's the heart of the issue. Your analysis of the situation is way off base. I'll tell you why.
The single biggest reason why NSMBU didn't match the sales pace of NSMBW is due to the amount of hardware that exists. Consider these facts:
1) NSMBW sold more than 10m copies in three months.
2) Nintendo was going to ship a maximum amount of 5.5m Wii Us during the console's first 4.5 months, because they wouldn't be able to produce more than that.
Therefore, even if you assume a complete sellout for Wii U hardware and a 100 % attach rate for the game, NSMBU would be capped at 5.5m units which is about half the pace of NSMBW. If you run these obvious numbers, it was always clear that it was impossible for NSMBU to sell at the same pace as NSMBW. It was impossible for it to do even half as well.
Instead of taking the obvious into account, you made up a myriad of other possible reasons. All of your arguments in this thread are based on an entirely false premise and the longer you ignore the obvious, the more ridiculous your arguments will get. You saw a problem where there is none.