My theory is they are only leaving for 2019 because they don't have enough games to show, because they decided to delay Death Stranding, TLOU 2, and Ghosts of Tsushima and make them all cross-gen, and they don't want to show them again until PS5 is announced and they can show the PS5 versions of them all. We will get a PS5 reveal event before the end of 2019 where they show the PS5 versions of Death Stranding, TLOU 2, Ghosts of Tsushima, and also announce some PS5 exclusives like Horizon 2.
But surely this is not the first time they've had little new first party titles to show at E3? They've been going for 23 years in a row, so I doubt they haven't encountered this scenario before. But they can make up for those titles with third party showcases, as always. I'm sure they can reach an agreement with Square Enix to have a Final Fantasy VII trailer ready for E3 for example.
The fact that they not only chose to skip E3 for the first time, but also said that they're looking for new ways to reach out to their audience suggests to me that this is more about them not wanting to waste millions upon millions on E3 when in today's world there are many other ways to reach your audience, for significantly less money, and with less factors that can go wrong with a live presentation, etc.
“As the industry evolves, Sony Interactive Entertainment continues to look for inventive opportunities to engage the community,” said Sony in a statement. “PlayStation fans mean the world to us and we always want to innovate, think differently and experiment with new ways to delight gamers. As a result, we have decided not to participate in E3 in 2019. We are exploring new and familiar ways to engage our community in 2019 and can’t wait to share our plans with you.”