I think there are really just 2 possibilities that seem likely
Either the game is in such dire need of a complete new change, may it be because they cant accomplish what they wanted to do or it didnt meet the quality standard,
Or they have another zelda spin off or hd remake by another company, seeing how zelda is probably one of the 3 series that are guaranteed to sell a million, in the works. So they delay zelda U since people wouldnt pay attention to a spin off if when the real thing is also there and use the extra time to improve zelda U.
If it is really ported to NX then not showing the wii u version does not make much sense, as this destroys their chances to sell some additional wii us to customers who are hyped for the game and even kill off a bit of the zelda u hype. And this would be counter productive as hype sells consoles at launch.
