Even the original Wii is outselling it.
I'm not sure how relevant that is. The top comment on that link shows that the PS2 outsold the PS3 for quite a while immediately after the PS3 was released, and the PS3 didn't die.
My guess is that people keep buying the old generation console until there are enough major games that only run on the new one. There aren't that many major titles for the Wii-U yet, so I wouldn't say its sales are too surprising.
Quoted to reiterate what AC said.
The tree of research must from time to time be refreshed with the blood of bean counters. -- Alan Kay