They could have done that in an special event, so why on E3 if this event is not important anymore?

With literally only one game streaming for 3 days it is going to be a disaster, especially since the treehouse team aren't that interesting of people. We'll hear "Wow, that looks amazing/beautiful/whatever" around a thousand times due to them not really having anything else to say about the game which will drown out any music, dialogue (supposedly), ect and we'll be wishing for the entire time they would just STFU for a while. 

because it's the Wii U version. Wii U isn't important anymore for Nintendo, they're already moving on, so they only give us something for Wii U and wait to show off the better version of the game in the actual NX announcement event...

Better no direct than an terrible direct like last year...

Seeing that they're doing a Treehouse, I guess they're trying to give a "live" and not rehearsed feeling to the game's gameplay.  With people around trying the game and all of that. The booth would be ZeldaU related, so maybe they're planning to do a cool "presentation", taking advantage of E3's affluence of people and a live presentation. They won't do a digital event, so I guess that's the intention. But we have to wait to see if it works or not. I'm not sure about Nintendo's intention, I'm just theorizing. We'll know in a month! 

They Wii U version mey not be important, but the game it's. So again, they could have dropped entirely from E3 and show the game in a more important event.

They could have done that live gameplay at their own event.

OK, guys, seriously, you need to read things more carefully.

Have a look - it says that Zelda will be the only "playable game". If they weren't going to talk about and show other games, it would say "only game", not "only playable game".

Besides, do you honestly think they'd be able to fill the ~8 hours a day of Treehouse Live streaming with only Zelda? It's fairly obvious that they'll talk about other games - those games just aren't going to be the main focus.

And I'm going to make a prediction: there will be a direct, it'll come at the end of the event, rather than the start. Why? Because it allows them to focus more on Zelda during the show, and then get good coverage on their other titles at the end.

Oh, I don't think they're going to do a live Nintendo event, it will be a digital event. That involves no people. E3 has people. At least that's my guess. They could do a live Nintendo-only Zelda event? Well, yes, but you could take advantage of the E3's own affluence of people without organizing anything. 

So, they could have take advantage for other things too, like new games or already announced games, even NX. This is not like E3 is suited for some things and other not. This event is important or it isn't, there's no middle ground. Our you go or you skip.

On the other hand I understand them. There is no reason to publish any big budget titles on the Wii U anymore. I mean, they forecasted pnly 800 000 consoles for this business year. The Wii U floped and they are aware of that. So it's better to focus on their next home console.