I have been googling about this and found this video
This one claims that the people who ran the studio system in the USA didn’t like the idea of revolutionary politics in the 50s/60/70s and that studios still don’t want to glorify the idea of “revolution” in general.
The argument he puts together is actually pretty good when he gets into the backgrounds of studio head honchos but I dunno.
Although when I think about it I don’t recall many movies about revolutions at all. Nothing big budget has been made about the French Revolution, or the Russian, or Cromwell in England, or in China, or anywhere…