My pet theory for a lot of this is that ubiquitous recorded media has given most commercial media an extremely long shelf life, and ubiquitous recorded media didn't really become a thing until about the 70s, so 70s music and TV has simply had more mindshare than anything before it.

The amount of media that continues to be popular today with people born after the media was created is directly related to how easily consumers could access recordings to rewatch/reslisten to the media, so while you might be able to name a couple of silent film stars from the end of the silent era, most people can't name the stars from 10 years earlier in the film industry. And every decade there's more iconic media until the explosion of iconic media with significant staging power from the 60s, 70s and 80s, a time when many Zoomer's grandparents were growing up