Can you tell me why popular films like the back to the future trilogy uhd and the et uhd haven’t been regrained like what you’re doing for Star Trek the motion picture’s directors cut?

I suppose it depends of people who make decisions, in case of TMP —>

https://www.blu-ray.com/movies/Star-Trek-The-Motion-Picture-4K-Blu-ray/294699/#Review
“This is discussed in detail in the supplements. Listen to the Fein, Matessino, and Dochterman commentary track at the 1:06:40 mark (as well as the “Return to Tomorrow” supplement) where the conversation begins about the grain removal and re-introduction because it was “distracting.” It detracted from the experience, they say.”

Some people think that “grain is bad”. And if it happens that the decision of what to do with particular movie is in their hands, then we may get botched transfers that doesn’t even resemble film look.