Psycho knows whats up.
If the Wii U sold like the Wii did I would expect to see higher sales numbers for Mario games. However, the Wii U suffers from iffy marketing leaving the super-casual audience who don't understand what a new console generation is confused. As for the "hardcore" gaming fans out there, the Wii U suffers from not being a Sony or MS console.
I'm willing to bet that in a decade we'll see the Wii U suddenly become a well-loved console in hindsight as people flock to its unique exclusives and wonder why they didn't play it back then, while they ignore whatever Nintendo's current console is.
Hmm... Sounds like the Gamecube. XD
That aside, I honestly do think that it may be due to when the Wii U was released. If memory serves, quite a few companies were unaware of the fact that the Wii U would be announced at E3, so of course they wouldn't have many IPs ready to come out alongside the console, since said companies were just as surprised as the fans at E3 about its announcement.
On the subject of Mario, IMO, Mario has been on a steady decline for a while, due to it usually playing off the Nostalgia Factor, rather than doing anything risky or new. The same can be said about earlier installments of The Legend of Zelda (to a lesser extent than Mario, mind you,) and a few other games in Nintento's clique. That being said, if you always pandering to the inner child of the consumer by introducing the same old, tired mechanics that show up in every installment without shaking things up a bit, of course people will eventually catch on, step back and take a good look at what's going on.
However, Mario 3D World DOES, in fact, shake things up and strive to be something different with the introduction of the Catsuit and the like, so I really don't know as to why it's selling less than its predecessor. If I were to make an educated guess, It likely boils down to what Psycho and Finalark are saying.