Game marketing no longer happens through magazines and legacy websites, but direct to consumer marketing (companies putting trailers on youtube and holding their own live streams) and influencer marketing (Twitch, Youtube, etc). The closest thing we have to E3 is The Game Awards, which is a 2.5 hour commercial that pays lip service to creators in the industry.
Games publishers big enough to have a catalog worth showing off at E3 realized that they would get more focused attention putting up their own show without letting eyeballs leak to their competitors, which is why you'll see Nintendo Directs and Sony State of Play announcements roughly annually.
Rather than compete for attention (and also rush to get a deliverable by E3 rather than taking the time to create something worth showing when ready), big publishers and developers were collectively like "why are we doing this? we can just show stuff off at our own event without spending time, effort, and money showing off something that's not ready? are we doing it just so the attention is on us and not our competition?"
That, and I think letting in fans was a mistake. No matter how cool it would have been as a fan to go to E3 during its heyday, it just wasn't the time nor place for that.