Its not the hardware. Its the variety of software.
There is no Wii Sports/Wii Fit type of title to bring in the masses.
There is no great variety of titles among first and third party to drive more common gamers and core alike.
Now you can argue 3rd parties backed off due to the specs of hardware... Fuck that. They backed off because of other reasons.
EA did so due to backroom negotiations where Nintendo did not implement the DRM we're seeing on PS4/Xbone. Some others wouldn't go Nintendo no matter what due to their personal issues. Warner Bros, Activition and Ubisoft are the only ones who actually only look at it from a purely financial point of view. They are bringing titles and not bringing others solely due to assumed ROI. Since Wii U is selling so poorly, they are not porting some titles. Had it sold 6m by March, Rayman would have remained exclusive and other titles beyond AC/COD/Watch_Dogs would have been announced.
Wii U is destined for 30m tops no matter how you slice it at this point. Its not going to drop the gamepad. Its not going to gain 3rd parties major support (though EA sports and other yearly titles will return), and its not going to be replaced super early.
Nintendo will support it through 2015 and then begin preparations for a new console.
At this point, I'm actually thinking this gen may end up very similar to PS2 gen. With PS4 being well ahead of Xbone and Wii U. However, Xbone will do better than Xbox and PS4 won't sell as much as PS2, but still over 100m... likely around PS1 numbers.