The problem the Wii U has is that sales are not increasing that much (if at all)
Take AC III and AC IV. AC III has sold around 230,000 copies, but AC IV has sold 130,000.
Black Ops II has sold around 220,000 copies as well, Ghosts has sold 140,000 copies.
The install base is going up but the demand for AAA third party titles seems lower.
As I say don't be surprised if no more Call of Duty games and Assassin's Creed games come to the Wii U.