I think the weak launch lineup Nintendo fears is in large part on the third party side. With a questionable 2016 user base and little time to work with the hardware, it might be tough to get 3rd parties entirely on board. If they reveal the final model later this year, developers will have a full year to port. They can also gauge consumer interest and have a larger user base for 2017.
Nintendo is not in the same place as Sony and MS. Third party support can not be assumed, and neither can mainstream consumer interest. They need to earn both, and earn it fast. A March 2017 release gives time to build hype and sell the concept of the machine.
It also means that another potential delay is possible if needed. If there is an issue with producing the hardware or if something else needs tweaking, they can do that and still hit Christmas 2017.
