The latter was only possible because it was originally shot in film, yes, but it was also an incredible amount of work. They needed to reassemble every episode from film!
They did a fabulous job of the Star Trek transfers (I’m currently watching the original series).
However, some studios put very little effort into their film to HD transfers. Last year, I watched Buffy, the Vampire Slayer on Disney+ and its transfer from film is woefully bad. For certain scenes, the picture quality looked like upscaled standard definition and these transitions were very jarring. Also, whatever filter they used for grain removal made the flesh tones and facial features look “wrong”.
Even worse was the wholly unnecessary conversion from 4:3 to 16:9. The resulting composition of many scenes was distractingly bad. At one stage, they the second camera unit can be seen filming the action from the side!
Edit: Fortunately, Disney+ have made The Simpsons (another transfer from film) available in 4:3 – as well as the default 16:9. There’s a setting in the UI to play it in 4:3.
I don't know about Buffy but some later Star Trek series like Deep Space 9 will be really hard to remaster because they were shot on video instead of film. The detail just isn't there to be enhanced.
Deep Space 9 was also shot on film. The editing, effects, and mastering were completed on video (same for TNG). This is true for nearly every 1-hour US prime-time drama of that era (Buffy, X-Files, etc).
I really wish for a ds9 remaster, which supposedly could be done in 16:9 since they were much more careful to not have mics etc inside the larger frame while filming.
But ds9 is not as popular as tng, and I think fewer of the effects where done on film, so it might bring in less revenue but may be even more expensive to remaster than tng.
Some people redid some ds9 battle scenes on YouTube and they look awesome. There’s also some remastered footage in the de8 documentary (I think “what u leave behind”?)
An this unfortunately also makes a remaster of VOY and DS9 very unlikely. I also read somewhere that they used more digital effects and 3D renders which makes a remaster even more difficult, as it would basically require to update or even redo them.
However, some studios put very little effort into their film to HD transfers. Last year, I watched Buffy, the Vampire Slayer on Disney+ and its transfer from film is woefully bad. For certain scenes, the picture quality looked like upscaled standard definition and these transitions were very jarring. Also, whatever filter they used for grain removal made the flesh tones and facial features look “wrong”.
Even worse was the wholly unnecessary conversion from 4:3 to 16:9. The resulting composition of many scenes was distractingly bad. At one stage, they the second camera unit can be seen filming the action from the side!
Edit: Fortunately, Disney+ have made The Simpsons (another transfer from film) available in 4:3 – as well as the default 16:9. There’s a setting in the UI to play it in 4:3.