The answer to why March could pretty well be software and hardware.
Software:
Even if they moved their unannounced games to the NX they still need to be finished and upgraded to the new console which takes time. For the 3DS and Wii U one of the biggest complaints the first year was there was "no" software after the initial push. And while this is the first time Nintendo has done a global launch in the spring, its not the first time they launched early
Hardware:
Up to the launch of the 3DS and Wii U things look like they were going well in production then they hit production snags after the launch for the gamepad. The extra time is probably to make sure they can have enough units to launch globally with out supply problems.
As for past consoles that launched in the Spring the PS2 comes to mind. They launched in the spring in Japan without any problems sales wise.
As for not showing at E3 that will not be that much of a problem. They are showing off Zelda which will be a launch games, even though they will be showing the Wii U version, plus they can have a reveal in August/September in the time frame of Gamescom and TGS, making the announcment closer to the release date and means they don't have to fight for as much for attention admist items that are being announced for the winter sales period.
