I think holiday 2017 is probably ear marked for the NX portable (provided there are two hardware lines). So they probably want to get the console out a few months before that to give it a few months on its own.
I don't think Nintendo really is interested in matching the specs on Neo/Scorpio. I'd be OK if NX is about as powerful as XB1 (not happy, but OK with it).
At least that would be a very good sized upgrade over the Wii U, whereas Wii was not over the GameCube.
You'd be easily able to run a game like Zelda: BotW at 1080P + 60 fps with added detail, better textures, better lighting/shading, higher poly count, etc.
For Nintendo's needs that's quite good.
March is not a great time to launch a console but if Zelda is ready (and it should be, it's been in development forever), and you have 2 other smaller scale Nintendo titles to go with it along with a few third party ports ... that's a good enough launch. Better to launch than just wait.