The reason officially cited for Colin Trevorrow leaving Star Wars: Episode IX was due to creative differences.
Now insights as to the creative differences look to have become known, which sounds as if Colin Trevorrow's ideas for Luke Skywalker and Star Wars butted heads with Rian Johnson and Kathleen Kennedy.
Enter Mark Hamill.
If you recall, Mark Hamill disagreed with Rian Johnson's portrayal of Luke Skywalker in Star Wars: The Last Jedi. Hamill even went so far as to publicly state that "he is not my Luke Skywalker," which is pretty huge.
I said to Ryan, I said, "Jedi's don't give up. I mean even if he had a problem he would maybe take a year to try and regroup, but if he made a mistake he would try and right that wrong." So right there we had a fundamental difference, but it's not my story anymore. It's somebody else's story, and Ryan needed me to be a certain way to make the ending effective. That's the crux of my problem. Luke would never say that. I'm sorry. Well in this version, see I'm talking about the George Lucas Star Wars. This is the next generation of Star Wars, so I almost has to think of Luke as another character. Maybe he is Jake Skywalker. He's not my Luke Skywalker, but I had to do what Ryan wanted me to do because it serves the story well, but listen, I still haven't accepted it completely. But it's only a movie. I hope people like it. I hope they don't get upset, and I came to really believe that Ryan was the exact man that they need for this job.
Mark Hamill even revealed the original J.J. Abrams ending to Star Wars: The Force Awakens was changed at the request of LucasFilm. It had Luke Skywalker in an epic moment using The Force with boulders flying around him; however, as Rian Johnson and Kathleen Kennedy wanted to essentially castrate Luke Skywalker in Star Wars: The Last Jedi, Luke couldn't be shown using The Force.
"When we were doing [The Force Awakens], Rian said, 'We might have boulders floating to show your Force emanating', so I was led to believe that I still had the Force and it was really strong in me," Hamill said. "When I read [The Last Jedi] before [The Force Awakens] came out, I said 'what?!" and called JJ [Abrams] or Rian [Johnson] to say, 'Are you guys aware of this? Have you seen a cut? Is there floating boulders?' And they said, 'No, we caught that and we worked it all out.'"
Now in an interview (watch below) with Mark Hamill that has surfaced, Hamill says he was on board with what Colin Trevorrow originally intended to happen with Luke in Episode IX.
The changes in the directors have been hard for me because I admire Phil so much. I admire Chris so much. I admire Colin, and their body of work. But again, it's got to be real difficult to come to a meeting of minds on something this massive. I had discussions with Colin. I was very excited because we were on the same page in terms of where we wanted to go and how we wanted to see Luke in a way that we never seen him. Even in this current version. But I don't know what went on. I don't want to know because there is no upside to that story. I like all those people. I like Kathy, and I like Lawrence Kasdan and all the people involved in that decision, but sometimes ignorance is bliss. And they don't tell me anything [laughs].
Obviously, if Mark Hamill is not on board with what Rian Johnson was doing in Star Wars: The Last Jedi with Luke, but did like what Colin Trevorrow - who was removed - was going to do with Luke in Star Wars: Episode IX, we can guess that Luke was going to survive The Last Jedi prior to Rian Johnson doing whatever he did and that it would have been a lot better than what happened with Luke in The Last Jedi.
From Mark Hamill offering they changed the ending to Star Wars: The Force Awakens, it seems as if Hamill signed on without knowing the whole story. While what Rian Johnson and Kathleen Kennedy did with Luke in Star Wars: The Last Jedi was downright awful, on the bright side, at least it got Mark Hamill back together one last time with Carrie Fisher.
A Redditor even chimes in (which obviously isn't verified) on the Colin Trevorrow situation:
There were two key things Trevorrow wanted to be changed - and they were not minor. These two things have been known for a while around Disney. Firstly, he wanted Luke to still be alive in 9. He strongly disagreed with killing Luke off in 8 when he had just been re-introduced. Luke and Leia were to be a pivotal part of his script, with Leia's Force affinity revealed in a much more brother/sister way that was apparently going to be "beautiful and tragic". Mark Hamill loved it. "We were on the same page," as he's said about Trevorrow's storyline. However, Kennedy and Johnson wanted Luke dead - period. Second, he wanted Snoke to live, as his back story was to be more discussed. Fisher dying scuttled his original plan for the Leia Force affinity reveal, of course. But he still needed Luke alive for the story he wanted to tell to end the trilogy - more of a Luke handing off the baton to Rey and a Luke "walking off into the sunset" idea, in the last film - rather than Luke dying in 8. They refused. He argued. They fired him.
And as noted, Colin Trevorrow seemingly took a swipe at Disney and LucasFilm by stating the new Jurassic World trilogy "is built for a new generation, but not at the expense of the fandom. Without them we’re nothing..."
True story: the DPG site is written by a team of JP fans who are now our creative partners. When I have questions about canon, they set me straight. The new trilogy is built for a new generation, but not at the expense of the fandom. Without them we’re nothing... https://t.co/Uiwr6DlEdt— Colin Trevorrow (@colintrevorrow) February 5, 2018
Mark Hamill interview: