The unused material would probably expose the film’s rushed development and lack of direction.
I definitely think it’s this. There are probably deleted scenes that contradict the finished story, as opposed to most Star Wars deleted scenes that were just cut content or alternate versions of the same story.
Like, remember the Finn vs Phasma alternate take that was deleted in TLJ? Imagine that, but Rose gets killed by Phasma, or maybe Phasma joins the Resistance. The fanbase would freak out and spend the rest of time complaining about missed opportunities (you know, moreso than we already do).