The Gamecube version of Twilight Princess still managed to sell over a million copies despite the Wii version getting all the attention and advertising. This is why Zelda is still coming to the Wii U, since it's still worth the money to do so. Wind Waker HD and Hyrule Warriors both sold over a million, so there's still a good sized audience of Zelda fans on the Wii U.
The only reason for Zelda to not come to the Wii U is if the hardware is too weak for their vision of the game and is causing major development issues that making it NX exclusive would fix and allow for quicker development that would save them more money in the end. Other wise if it's just like Twilight Princess where they need more time to finish the game, there's no justification for moving it to the NX when they can make more money by releasing it on both systems.
Seriously, the Gamecube version of Twilight Princess sold 1 million while the Wii version sold close to 7 million. Combined Twilight Princess is the best selling Zelda game of all time so releasing them on 2 systems at the same time clearly wasn't a bad thing for Nintendo. That game showed they can release the game on their newest system to appeal to a larger audience that didn't buy the previous system while still pleasing the hardcore fans who own the old one.