Here's the thing;
Mario Kart and Smash Bros are two of their most popular series of all time. These are the two games they are the most hyped up about, and these are the two games that Nintendo has actually given a time span for launch (Mario Kart May 30th, SSBU Winter). X and Bayonetta 2 (being very far outside Nintendo's usual child friendly audience) will probably do rather poorly on a console that doesn't want 3rd party games. While Super Mario 3D World did fairly well in recent times, so many 3rd party developers are leaving the console in the dust because every game they make for the damn thing never sells over a million units and doesn't make its money back.
That being said, the Wii U needs somethings to revitalize it's stability and try to regain the casual market now stuck on tablet or other devices. 3rd party ain't gonna do it, so what's left is the 1st party games that are in their fourth and eighth installments (SSB and MK) that tend to grab the most people from most other consoles (and particularly the Wii).
Oh, and I suppose you can thank the people who constantly go on forums and tell others to "Wait for MK and SSB to come out before saying the Wii U is bad."
