I don't know if this is the reason behind your problem, but I've found this article.
The Wii U version of the game, which is also coming out on the 3DS,
PS3, Wii and Xbox 360 consoles, will have higher texture density and
screen resolution than its other console counterparts.
Skylanders: Giants still has players using a Portal of Power to
transport plastic figures into the game as playable champions. Those
plastic figures, some of which no light up, save the champions
progress so they can be used on any platform, or account.
While the Wii version of the Portal of Power used a wireless USB
receive to communicate with the console, the Wii U version will use a
Probably this is why the Wii version can't work on the Wii U, they use a different communication system.
So, yes, you may need to buy the Wii U version of the game.
But luckily the Wii version of the portal, as well as all figures, will still work with the Wii U version.