I think they're waiting to sell a good bunch of Amiibos to release the game. So, they've already sold a lot of them. Now it's the time to release that game, and I'm almost sure it'd be a NX game as a launch title to push hardware sales. That's the logical way of thinking. But, yeah, it's Nintendo so, who knows?

Sad that they've become so predictably stubborn that we even have to debate this. But after readng the comments in this thread, I'm hoping for their sake that having this on the NX at launch is a no brainer for them. Also, someone pointed out how Amiibo only came out about a year ago. It should take 2 years on average to develop a good quality game like Skylanders or Disney Infinity. If they waited to monitor Amiibo sales for a few months before greenlighting such a game, the game couldn't be ready yet

Yeah, I think that's the case. First, wait to see if Amiibos have sucess. Then the next step is to create a Nintendo's answer to Skylanders and Disney Infinity. But it would take them around 2/3 years to make a decent game. And the game could sell like hotcakes, and I think it would work as a system-seller for people who have bought amiibos but didn't jump to the WiiU. They made some dumb decisions, but not making this game would be another level of dumbness. If they manage to release the next console with a brand new 3D Mario, a Nintendo's Amiibo game and a new Retro Studios game (which I think it's reasonably realistic) they are up to a great start. 

Nintendo wont.

This just feels like one of those times where Nintendo knows what the fans want and knows their idea will be successful but they just simply wont do it.

The same reason they haven't made a console/mmo Pokemon: hurrp durrp!

Nintendo won't make such a game unless they can bring something new to the table. Making an amiboo game wouldn't be enough for them, they would put their own spin on it. Just look at Splatoon; that's pretty much a shooter, but it's not an ordinary shooter. Untill they figure out what to do with an amiibo game they simply won't make it.

Yeah, Nintendo for the last 5 years had this weird thing where they seem to dig their heels in and refuse to do the sensible thing, like a child that won't do as he's told just to be defiant.