how were these communities so isolated for so long?

Have you ever gone camping for more than a week? Your priorities are food, water, shelter, warmth, and keeping yourself entertained. You're not worried about the latest political scandal or whatever else "civilization" cares about. You simply live in your own little bubble of concerns that does not willingly seek out information about the world outside these basic human necessities.

Rural, 1960's China (as with much of the underdeveloped world) is like that. You wouldn't find it hard to believe that there are some remote villages of people in the Amazon that still probably don't know who their government is. It just isn't their concern.