Everything was recorded on 35mm film originally, then scanned to tape with a telecine where the editing and effects was done meaning a huge loss of quality. The original film has far more detail than this picks up and can be re-scanned in HD/4K, it's just a huge amount of work needed to find the right footage for what was used in the original edit, scan it all and redo a lot of the sfx. It's been done for all of TNG and TOS, and even a few scenes of DS9 for the "What We Left Behind" documentary a few years ago, they just never committed the resources needed to do it for all of DS9.

The community done upscaling efforts are cool, one of the better uses for machine learning out there for sure, but there just isn't the information to reconstruct all the detail a real remaster can pick up.