Nintendo comes after two failed launches. I don't think Nintendo affords to hold things back. A console is a long term investment for most of the people, is not just about the launch line-up. They need people to buy it also past the launch month. The January event is not just for the fans like us.
I don't want to see again a panic move like the January 2013 Direct. Nintendo needs to be proactive not just reactive.
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.
On top of all that January is just not long enough after Sun and Moon's original release. I really can't see a situation where a third version is announced for the Switch on that short notice and doesn't create controversy. They're going to wait long enough so the announcement can come at just the right time when the "but I JUST bought that" period is over and people are legitimately interested in something new from the series.