They are gearing up to the PS5.
I don't think Sony is ever coming back to E3 (or at least for some years, if their new models goes wrong), they said they are looking for new ways to engage the community, there is nothing to do with "Sony has nothing to show" as some people are trying to push, there is always something to show, even if its nothing theirs, PS4 has a huge amount of 3rd party support and could bring several announcements from them, they have never missed a conference before, this is unprecedent, Sony has left E3 for good, the statement was pretty clear IMO.
The point is Sony is using the PS5 reveal as a chance to promote themselves even more, PSE will be their whole focus, and they might be thinking about spreading their announcements over the year.
E3 was a big show and good for Sony but now they want ever bit of the attention they bring to benefir only themselves, and they'll use the PS5 to catapult this.
From now on PSE is their main event, they might still appear on small events here and there if the timing is good, but the big guns will all be announced on events of their own.