The focus of the event is a big part of it. GDC is a developer conference, and showcasing games in development is very small part of it. PAX is an event for gamers and while a lot of games do get showcased, it's not the main purpose of the event.

Traditionally E3 was about developers showcasing games for the press. But that's always been a bit of a tough sell for a major event, and I'm not surprised it's struggling given the current media landscape.

Possibly, the ESA relied on it for revenue. Though PAX is run by a for profit event company, I think.

PAX is better at managing a public event. E3 wasn't originally open to the public, and once they opened up it was never done very well. If publishers want to have that public convention thing, PAX is better. Letting the public in to E3 sort of ruined the industry insider aspect of the convention, all the meetings moved to hotel rooms so that people could hear each other.