Why was not this shown during State of Play you ask? Probably because the same people who were responsible for picking and choosing the content for Sony's E3 presentations are still responsible for picking and choosing content for SOP. Throughout this whole generation, Sony has omited many of their first-party stuff from E3 showcases, just to relegate them to pre-show, or a blog post and youtube video a few days later. Gravity Rush 2, Gran Turismo Sport, Knack 2, etc. I think even Ratchet & Clank 2016 did not make it to the big show.
New ways to announce and present stuff, still same old unexplainable omissions.
The best explanation I've seen for why it wasn't included is that this is an early access version of Dreams. You don't want your average joe PS4 owner getting it and there's not much content so they get pissed off and trash the game. Media Molecule wants this version in the hands of their dedicated fans who know what they're in for. It was the right decision.