According to Johnson, it was "never in the cards".
This, to him, seemed to be "always the plan", and he's just exited to work on the new trilogy he had pitched and was set to direct.
It was never in the cards. It was always the plan for me to do this, and then hand it off to the next director, whoever that was going to be. So, I never had that really in my head, and now I'm just really excited about the next trilogy.
Cinema Blend - How close Rian Johnson really came to directing Star Wars Episode 9
According to this Slash Film article, The Hollywood reporter caught up with Rian and asked him about Episode IX getting the following response.
J.J. Abrams is doing a third movie. I’m not involved in it. They’ll be writing their own story but continuing on with what we did.
The Hollywood Reporter - Interview with Rian Johnson
In a statement he'd made in September, he follows the same sentiment
“It was never in the plan for me to direct Episode 9, so I don’t know what’s going to happen with it… for me, I was entirely focused on Episode 8 and having this experience. Now I’m just thinking about putting the movie out there and seeing how audiences respond to it. So no, I’m not really thinking about that right now. Whoever does it, I’m going to be really excited to be an audience member again, and to sit down and see what the next filmmaker has to show us and where this story ends up going.”
JJ Abrams was announced on the 12th of September 2017 to be coming back to reprise the role of writer and director for Episode IX
With The Force Awakens, J.J. delivered everything we could have possibly hoped for, and I am so excited that he is coming back to close out this trilogy
Kathleen Kennedy - StarWars.com
According to this vulture article, the reason for Trevorrow's departure was due to creative differences/personal differences with Kathleen Kennedy. Supported by this article on StarWars.com (apologies for the fruitful language)
“There’s one gatekeeper when it comes to Star Wars and it’s Kathleen Kennedy,” says a veteran movie producer, who has worked with the studio chief. “If you rub Kathleen Kennedy the wrong way — in any way — you’re out. You’re done. A lot of these young, new directors want to come in and say, ‘I want to do this. I want to do that.’ A lot of these guys — Lord and Miller, Colin Trevorrow — got very rich, very fast and believed a lot of their own hype. And they don’t want to play by the rules. They want to do shit differently. And Kathleen Kennedy isn’t going to fuck around with that.”
Vulture - Colin Trevorrow’s Firing From Star Wars Is Another Reminder That No Director Will Ever Be Bigger Than the Franchise