Launching Holiday 2016 just puts them lost in the fray of the VR jerkoff and PS4K.
Holiday 2017 means ports competing with existing console dev time and supply constraints
March 2017 (and Nintendo has launched multiple successful hardware in March) gives them an E3 between launch and holiday, gets them out of the 2016 clusterfuck, lets them account for the 4K if they so desire, developers aren't making the NX version of late 16 / early 17 hardware at the same time as other consoles, they have a GDC, PAX East, E3, PAX Prime that they can openly have hardware and playable demos at.
March 2017 also gets them Destiny 2 in September 2017, instead of Destiny 2 Months After It Already Came Out.
Ubisoft has paused AssCreed for 2016 and won't be releasing one this holiday, so Nintendo isn't missing out on a major Ubi title either for not making this year.
*developer hat on* Strategically, this all makes sense. It makes sense for ports, it makes sense for hardware supply, it makes sense because they can let Sony talk themselves ragged for E3 2016 and then be the main attraction for GDC, PAX East, E3 2017, and PAX Prime going into Holiday 2017.
*developer inside baseball hat on top of developer hat* Also, by releasing at the start of most everyone's fiscal year, they won't suffer from what happened to the Wii U, where a bunch of companies pushed U titles back into fiscal 2013 to make their numbers look better. Developers spin down during December and January, so why put the NX in that position anyway.
Plus they can make retailers happy by exhausting all of their Wii U supply this holiday. Not gonna bet money but wouldn't be surprised to see a farewell Wii U bundle that acts as a soft price cut to empty out the warehouses and retail channel.