All of the 90s Star Trek series were filmed on 35mm, but all of the post-production work (editing, SFX, etc) were done on tape, at 480p. There's no 35mm copy of the final result to scan and color-grade for a "normal" HD remaster. For the TNG remaster, what they actually did was re-do all of the post-production from the original 35mm negatives. VFX were re-composed, some were re-done in CG. They finished the TNG remaster just as streaming services were ramping up and Blu-Ray disk sales declining, and sales were disappointing given the amount of work that had to go into them.
Paramount will never remaster DS9 or VOY because they don't expect to make the money back, because neither was as popular as TNG. And it's worse for DS9 and VOY because they extensively used CGI effects for things like ship battles, which were originally rendered at 480p. If the original assets could be found, they could be re-rendered, but in many cases, they would have to be fully re-created.
Fans have created AI upscales which are generally better than watching the horrible 480p releases that were on Netflix and are now on Paramount+. But they are also sometimes uncannily smooth and unpleasant.
