The main reason why no-one's buying the system is that there are no games for the system.
Despite all their claims that they'd learned their lesson from the 3DS, Nintendo went into the Wii U launch with no games for months afterwards. It shouldn't have been this way, but Nintendo delayed pretty much all of their titles (or scrapped them entirely - see Scribblenauts in Europe) and we've lost one of the biggest titles of the system's touted launch by Rayman being pushed back to the end of the year.
The fact is that the Wii U still doesn't really have anything going for it. There are no hard and firm dates for any of Nintendo's own titles and nothing has really been announced since launch other than the WindWaker remake.
It's taken until this year, 2 years after the 3DS came out, for the quality games to finally start coming for it. It's seeming more and more likely that the Wii U will follow the same pattern.
This is incredibly destructive, though. The 3DS is basically in a league of its own as its major competitors are mobile devices - but they'll never match up to the full 'premium' gaming experience that the 3DS can offer. The Wii U, however, is already pretty underpowered. It's a major step up from the Wii, and a bit of a step up (in some regards) to the Xbox 360 and PS3 - but none of those are the system's actual competition. The real competition lies in the new Xbox and PS4 that will hit this year - seemingly long before the Wii U actually gets any real support.
How can third parties justify the excess cost of downgrading a game for the Wii U when Nintendo can't even get the sales of the hardware itself on a consistent level with their own titles?
The system can't support the new major game engines (Unreal 4, Cryengine 3, Frostbite 3) that most AAA games on the Xbox, PS4 and PC will run on, and worst of all, it's extremely likely to be the only system running on the PowerPC architecture from now on (everyone else will be running PC-like x86 architecture), which will make porting everything to it far more difficult than making multiplatform titles across the three other major platforms.
It's looking increasingly bad for the Wii U being able to garner third party support - and that's going to lead to the exact same problems the Wii suffered from, only this time, without the massive sales that the WIi managed to achieve through a perfect storm of media hype and price.
The Windwaker remake alone is enough to make me happy I own a Wii U, so I won't regret my purchase. But I can't really see a bright future for the Wii U unless Nintendo gets their wallets out and starts making deals. We're already looking at a year where none of the main multiplatform games will be on the Wii U - and there are a significant amount of massive multiplatform titles this year.
Honestly, I feel Nintendo have made a massive error of judgement in the Wii U's specs. With sales as poor as they are, I don't think they can really expect (or afford) to keep this going for a full generation, either. If things don't pick up within the next year, shareholders are going to demand action, and that could mean an early death for the Wii U.