Because it appears to be the next Super Mario Bros. Extremely high selling, universally regarded series (yes, everyone loved Wii Sports besides the "serious gamers" and people constantly ask me if there's a Wii Sports U on the way) that was neglected by Nintendo due to their unwillingness to develop another one.
Seriously, a Wii Sports U with online multiplayer and *some* gamepad support would sell extremely well. But I guess they just couldn't force the gamepad into every single game so they probably scrapped it (remember, we even saw a demo at e3 2012 (or 2011, can't remember). I haven't lost all hope yet, personally. Wii Fit, Mario Kart, Art Academy and Wii Sports in 2014 would be a massive push towards the mass market for the Wii U (if the gamepad didn't exist... oh well.)







