The declining relevance of E3 is not just a Nintendo story, it's an ongoing one throughout the industry. Activision and EA have declined to book booths at the event this year as well. There's legitimate discussion to be had on the future role of the expo and its diminished importance in the eyes of publishers when multiple avenues, including direct ones, exist to better shape and control the messaging and exposure of announcements.
Why do people think e3 is for the masses and the press?
It is where business gets done. There is no replacement for it, because no way are non-indie business guys going to go to a PAX sweatbox and have the hoi polloi annoying the shit out of them.
Just because Activ and EA have decided to do their own presser for the masses + enthusiast media doesn't change e3's relevance.