I don't think Nintendo would want to look lackadaisical in contrast to Sony and Microsoft by not holding a proper press conference.
I don't think they'd ever stop having them anytime soon, but I think this E3 specifically highlighted that these conferences are only just one media event in a year full of other oppertunities to show off new products. The big three console companies don't treat the event as "Gamer Christmas" anymore; with all three of them spending nearly all of their time on games that were previously announced or known about. It's a week full of noise, and you're more likely to get noticed if you announce things separately on slow news days instead of in the mess of that event.