Xenoblade X bundle is a limited bundle that already has its pre-orders closed at retailers (long time ago). It's not like the MK8 bundle that became a standard bundle. This is limited, will sell all copies FW and just to the ones who pre-ordered the game. Users aren't holding to purchase the Wii U for the bundle, maybe they're just waiting the game to come out. But this doesn't justificate the current low hardware sales.
That said, I wouldn't be surprised if Xeno X sells better than the original. And since is released in Golden Week hardware sales should be favourable.
Media Create Annual Archive (Spanish version)