so a few scenarios i'm thinking happened:
1. oh god panic everywhere nothing's ready. mario's not ready. retro's new sleep training device isn't ready. somehow zelda's not ready either. the nx launch was just going to be lego tales of minecraft and a splatoon update that included all dlc. so they're rushing projects to meet march and they're pushing the hardware back to meet those projects. (see also: 3ds launch)
2. spend money smarter. maybe with ps vr this holiday and some things nintendo's aware are happening in late 2016, they decided to vacate the premises and focus on an even better launch than what they were planning. with their marketing dollars out of 2016, they can buy less expensive ad space in america, and they can focus better on a worldwide launch when that wasn't happening before. maybe a handheld component is happening at the same time, or set for november, and having launches closer or at the same time will benefit the handheld form factor better.
3. shit happened and a delay happened out of their hands concerning the manufacturing of some of the parts for nx. so there really isn't going to be anything ready for e3 because they can't get the special tech working or even approximated in time. november would be a gamble, but march is a sure thing as far as those components are concerned, so the system is pushed back to that date to at least guarantee there aren't shortages or other issues on release.
kinda going with #1 myself, but that's after knowing what happened with wii u.