Don't you think it would be key to have the January event be the beginning of strong momentum which is supported by Directs throughout the year and an excellent E3 showcase? If Nintendo announced absolutely EVERYTHING in January, and then showed up at E3 going, "nope, that's it, we have no more announcements for the whole year" that would instantly kill their vibes they had spent so long to build up. Bad move. It's important to time announcements and marketing strategically.
Even at their E3 this year they managed to have completely new announcements despite their anemic lineup. Inspiring consumer confidence is a long-term, continuous process.