That is a difficult question. Many have brought up the lack of third-party titles for the system, but the reasons for that lack are manifold:
1) The WiiU is difficult to program for. Many of the early ports of third party titles to the system were reasonably poor, like Call of Duty and Batman. This is more to do with the fact that the WiiU is much like the PS3 in that it is a machine with design difficulties. Take the reliance on eDRAM4. This makes the machine faster, but also makes it different than PC and Xbox 360, and developers looking for a quick cash-in on their ports will take numerous short-cuts and end up with an inferior title.
2) The WiiU is a powerful machine compared to the Xbox360 and PS3, but the developer tools for third parties are lacking. Nintendo has not made many advanced development tools available to third parties until recently. This was perhaps because the firmware updates and other things took priority for Nintendo in the early moths, but it left developers with a somewhat limited toolbox.
3) Why on earth was it called the WiiU? I disliked the name of the Wii rather strongly, and I think this makes the console sound, to the layman, like an expansion of the Wii rather than its own console.
4) Third parties have used it as a whipping-boy for the state of the industry. Rayman Legends, whatever Ubisoft said, was not delayed because of poor WiiU sales, it was delayed for the exact same reasons Splinter Cell Blacklist was delayed: to bump up Ubisoft's FY2014 Q2 earnings so they did not lose money over the marketing push for Watchdogs and AC4. Basically, from calendar years April 2010-September 2013, Ubi was 90 million euros in the red. Investors were getting anxious, but they made it back with AC3 and surprisingly strong sales of Far Cry 3. However, to lose money even in a single quarter would send the wrong message to investors, they pushed back first Splinter Cell, then Rayman to cover their marketing campaigns for Watchdogs and AC4. So, the WiiU ended up being the whipping boy for Ubisoft's poor management in that case.
There are many other reasons as well, but both a difficult machine and poor circumstance are to blame. Though, remember that the 3DS started off even slower than the WiiU, and that is now one of the best game systems out there, making Nintendo money hand over fist, and having some truly great game as well. So, I am not sold that this console will be Nintendo's death knell.