According to game informer
As the industry evolves, Sony Interactive Entertainment continues to look for inventive opportunities to engage the community," Sony said in a statement to Game Informer. "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
Based on the things sometimes discussed on the Bombcast this seem like it was a long time coming in terms of E3 not being needed as much but its still A very interesting announcement.
I think companies just don't want to fight over bylines or having to keep big news until a certain date or announce something when they don't want to. Personally i'm fine with it as we probably don't need more last of us footage or death stranding in my opinion.