There have beens signals that Trevorrow wasn't working out, and there was never any enthusiasm for him, so this may be a good thing. It's certainly better than getting all the way into production and realizing it's not working.
I think as Nicklab says the films so far have lacked a cohesive vision. Kathy Kennedy isn't a visionary filmmaker, but she knows what a a SW film and has 3 billion dollars to back her up. What the films need is a guiding force, within the context of the films, and that person seems to be Rian Johnson. He gets it. There's no issues with TLJ and he's most likely going to take over IX. I think that's great. What the films need is stability and continuity.
Also, I don't think Star Wars is as elastic as some, especially Kennedy, might have believed. Coming into this, everyone anticipated all kinds of possible movies - a war movie, a heist movie, a comedy movie. SW has been a synthesis of all these films and genres. When you lean on one, it suffers. I think a SW film is pretty much what we've seen. There are variations in tone, but not in genre. That bodes ill for future anthology films, and off the shelf directors. It has to be within a certain context, guided by a writer and or director who gets it. Not just knows it; they get it. It comes from a place of understanding exactly what these movies are.