They system is too weak and not supported by most modern engines.
It is expensive to develop for another architecture (PowerPC) if all other plattforms (PC, PS4, X1) use AMD64.
The Wii-U audience is too young and does not buy 3rd party games.
The Wii-U user base is too small.
Nintendo's own games are too good and many 3rd party games cannot compete, so Wii-U owners buy Nintendo games instead.
The Wii-U has not been successful and might get discontinued very soon.
The Wii-U is heavily overpriced, so userbase growth is very low and many Nintendo fans or Wii owners refuse to buy the system.
Development costs do not pay off as all previous attemps to develop 3rd party games for Wii-U failed.
If you develop for Wii-U as a 3rd party developer you loose money istead of making it.
It is now to late for a turnaround, Nintendo management is to blame for the current situation. A tablet-less Wii HD with competitive hardware, Wii Sports HD and HD Wii game support and AAA 1st (Mario, Zelda) and 3rd party (funded by Nintendo) adult games on day one after launch would have been a huge success.
