[NickC25]

I think this is the big one.

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.