Consumer confidence in the Wii (as a brand) damaged when Nintendo abandoned the console in the later years. Terrible marketing at launch/post launch - people to this day still don't understand that a Wii U is standalone console. Gamers disininterested in ports of games on the console, when they can already play it on ones they aleady have and are not a big leap from last gen console versions.
Then there's the lack of truely compelling games.. Its lineup lacks variety and punch (to even the GC) and some of the more important third party games are noticably non present for the mainstream audience, or have no presence on that platform to begin with. It's also competing with the 3DS in first party content, and loosing at that.
Not to mention the price.. $300 is expensive for a Nintendo console. Not many people outside of their current userbase are willing to buy a Wii U, for that price to play some Mario. And, due to the added BOM of the Gamepad controller they can't exactly drop the price of the console either. So what we have here is the console for hardcore Nintendo gamers that are looking for more Mario and Zelda (with promise of other titles), among other titles. The rest of the people who want a machine to play Nintendo games need to look no further than a 3DS to fulfil their needs.
Also, cold hard data.
