[iClaudiusX]

I would trace it to a few factors; most important is the rise of the influencer advertising strategy.

It is vastly cheaper to simply pay streamers to hype up your game on Twitch or youtube than it is to give an honest, sober demo live on an E3 stage. The audience at these press events has shifted from enthusiast reporters/critics to hyperventilating influencer personalities. It was so bad this year that they were obnoxiously screaming and interrupting the presenters every few seconds to the point where the people on stage were losing their train of thought.

The other aspect is that putting together a demo takes time out of development. Almost every game shown this year had a release date of either Fall 2019 or Spring 2020. That means they're either on crunch or ramping up for it and can't afford to set aside a few months to make a vertical slice for E3. This is partly due to the end of this console generation with new hardware coming next holiday season.

The longer term trend is that marketing has caught on to the irrelevance of E3. They can run their own Nintendo Direct style live stream whenever they want to speak to their audience and the press will disseminate that info to the wider community.

You're even seeing companies like EA experiment with dropping a new title with zero advance notice as they did with Apex Legends (their take on battle royale, from the Titanfall developers). They just had an influencer preview event the week before and dumped it to the public with pretty wild success.