No games. That's hyperbolic of course. I actually have a Wii U, and I like the games I have for it. But speaking from a former outsider, that's what prevented me from buying in earlier.
Specifically core, new, first and third party titles. Wii U has no killer app. NSMBU is nice, but I think a new 3D Mario would have done way more for the system, especially considering NSMBU is essentially retreading the same old ground from the rest of the "new 2d mario" series. A new Zelda, Metroid, Donkey Kong, etc. Those are the kind of titles that sell Nintendo systems, not minigame collections ala Nintendo Land.
Another massive problem was with the marketing. Immediately after the first trailer at E3 a problem became evident in that people didn't understand that it was even a new console. They pushed the controller so hard they forgot to sell the system itself. Now they realize that, but they should have much sooner.
And the fact that third party developers and publishers are creating a vicious cycle by being reluctant to publish titles on the system because of poor sales - when part of the reason it has bad sales is lack of third party titles.
Those are the key factors I think contribute to its slow start.
All that being said though, Wii U isn't really doing bad. It's not selling great, but you've also got to consider that they're making more money on those few sales than they would on the Wii, and are therefore in healthy financial standing: http://www.escapistmagazine.com/news/view/126475-Nintendos-First-Quarter-Shows-Rising-Profits
And considering there's a number of solid core titles on the way from Nintendo and third party devs in the future, I'd be willing to wager that the Wii U will slowly gain sales by building its library over time. The 3DS similarly had a somewhat sparse opening library, but sees greater profit now that it's had a bunch of great releases in a short time span.