many of those progressive things are [now] either the norm, or seen as regressive
Totally agree.
Part of it might also be that they didn’t see Trek as anything more than “cool space show, with a whole bunch of scantily clad men and women”, and didn’t bother to look any deeper
Again, I think we're actually in agreement. If you look past the cool space show and can avert your eyes from William Ware Theiss' gravity-defying outfits you should be able to discern that Roddenberry's future is largely socialist, some would argue even communist. Centralised world government, no private enterprise (pun not intended), and by the time TNG aired, even no money. (Note there were references to money in TOS.) Not that I'm trying to imply conservative Trek fans aren't smart enough to figure this out. But - like the diversity and inclusion in the TOS cast - TOS's liberalism (social, not economic) isn't something that the show hit you in the face with. It's treated matter-of-factly, as backstory or backdrop. Whereas a show like DIS basically grabs you by the lapels and shouts "I'm progressive! I'm progressive!!" (Exaggerating of course, but you get the idea.)
not unlike Star Wars. It’s just guns, cool ships, and shooting, with the imperialistic allegory being ignored, or gone unnoticed
Not much of a Star Wars fan, but I assume this is David Brin's critique?