There is eveidence based on their past. When TP was announced, the game wasn't even being made yet. The 2004 trailer was just made for the presentation, and Aonuma literally said in an interview years later that he had no idea what the game would be until literally two years after that. And then it was delayed again purely for it to be a launch title for the Wii.

It's PR speak, because it sounds better than "this thing was never realistically coming out this year, and we announced it way earlier than we should have."

Hense my annoyance. 

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.

This is the most likely explanation, by far.

The easiest sollution is usually the easiest. Zelda U won't be avalible this year because Nintendo can't finish it in time, maybe because of the game being too long, developement problems or the need for a lot of testing.

That doesn't really explain why they can't even show a trailer. Can't make it for 2016? So what? Xenoblade X didn't make 2014, yet Nintendo still showed an extensive trailer for it for the last two E3s. 

Something is up, either there is some platform switching tomfoolery at work here ala Twilight Princess (which is what I suspect) or the game has undergone some other changes, but that might actually be a worse scenario, because if they opted to basically scrap whatever they were working on before to start anew, we might be headed into Final Fantasy XIII Versus (now known as Final Fantasy XV) territory. 

If you can't show a game after 4 years in development ... something is up. 

My shot in the dark was with number one after I heard about the delay.. Not showing it at E3 could possibly be that they want to amp up the graphics... though it's rather unlikely since Nintendo commonly delays games.

Honestly I would laugh so hard if the real reason for the delay is a complete overhaul in art style to a "realistic" direction. This would mean the same people that whined about not having realistic graphics that are now whining about the delay would be the ones responsible for the delay. So they can have what the want. Lol.

Game is freaking huge, especially if compared with previous Zelda games,
they cant just finished this year if we don't want big empty world.

I wouldn't be surprised at all if the title is no longer a Wii U exclusive, just that publicly, Nintendo is unable to discuss that aspect of the delay until the NX is unveiled to the public.

I would be shocked if Zelda came out before holiday 2016.