It's hard to find all the titles at E3 by walking the show floor. A company's press conference serves to give you an idea of the main headliners to look for. It also serves as a grandstand for you to boast and draw attention AWAY from other companies.
Not just for gamers and the press, but DEVELOPERS, PUBLISHERS, and RETAILERS. You know, the people you get your games FROM. E3 is where places like Target, Wal-Mart and Gamestop (Aside from Gamestop's own annual party in Texas) talk to companies about what to order, stock and such.
If a Nintendo Direct was enough to work around E3 time, then they'd just make a Nintendo Direct the same week as E3. They chose not to. Or maybe it's because out of desperation with the last two Directs they don't have enough to show to fill 45 minutes.
Or maybe it's because their usual presenter, Reggie, is on his way out.
Or maybe they saw the futility in trying to compete with the new juggernauts.
Heck, all Sony and Microsoft need to do is show a 2 minute trailer and publish a pdf with the specs on it with launch date and price.