You know, after licensing costs and some marketing costs (like putting the game into box, and shipping the game), I'd expect that they made $10 million in revenue for AC4 Wii U. While I doubt the costs of porting are larger than this, it simply isn't worth it to port.
The problem with Nintendo platforms is that audiences seeking "mature" experiences are largely elsewhere. Even if they do buy a Nintendo console, chances are they'll play it only for Nintendo games. Also, if the first game you introduce in a series is horrible, you'll have to double the effort in your next release to lure the Nintendo faithful. That's just how it is.
Multiplats don't sell on Wii U. (Largely because definitive experiences are found elsewhere; Unless, of course, that it somehow caters to the Nintendo audience more than others.)
"Mature" titles don't sell well on the Wii U. (Largely because of the audience; ZombiU is an outlier.)
Ubisoft has opened yet another Pandora's Box by burning bridges with Nintendo's faithful.
I WON A BET AGAINST AZUREN! WOOOOOOOOOOAAAAAAAAHHHHHHHHHHHHHHHHHHH
:3
