Why would Smash or Zelda be able to sell more WiiUs than MarioKart?
WiiU needs the boost now and it needs to ride on it for a while. There will not be another big game for at least 3 and a half months. That's if they announce something for September at E3. And what could they possibly put there much stronger than WWHD last year? Nothing we know about yet.
Marketing's the only other thing WiiU can ride on till November. I just think if MK8 and marketing can't do it then nobody wants the hardware and Smash+Zelda won't make a bigger splash.