Nintendo can't announce NX formally because the Wii U and 3DS are so weak that their sales would crater.
They could show NX today I'm sure if they really wanted to, they probably could have shown it at E3 earlier in the year really. But they've never had a platform as weak as the Wii U (aside from Virtual Boy) and it would cause Wii U sales to collapse and would not help 3DS either. They need to get through this holiday season which is probably going to be one of their worst (in terms of hardware sales) ever, making a bad situation worse by showing off the next-gen platforms early than need be would just be stupid.
That's the main reason they're not doing it.
Beyond that it's 2015, not 1996 anymore .... debuting a hardware platform 18+ months before release doesn't excite anyone anymore. Like who wants to see a cool piece of tech and then be told "now wait 18 months for this, ok, thanks bye".
Yeah no way. PS4/XB1 were unveiled 5-6 months before launch for a reason, Apple releases their iPhones/iPads/etc. soon after unveil.
The electronics world has changed, and why would you want to do anything like the Wii U again anyway? Pretty much nothing went right for that console.
