I seem to recall MH3U and Lego City Undercover increasing Wii U sales in their respective launch weeks by a factor of three and two, respectively. That said, Wii U sales were pretty dismal at that point (hanging around the 20,000 units per week mark), so it's not exactly something to go nuts about.
The problem with third party multiplats for the Wii U is that the majority of the people who are interested in said third party titles have already long since bought a console to play them on. Since the Wii did a major number of Nintendo's reputation with fans as far as third party titles go, most people who routinely buy those games have a 360, PS3, or PC to do so. Nobody's going to buy a Wii U just to play Injustice on when they already have a console that will play the game fine.
It also hasn't helped that the Wii U versions of multiplat games have been getting gimped pretty badly compared to other consoles.
