The reason is simple. The Progressives controlled the media, for all practical purposes, for about 35 years, 1960 to 1995
I don’t know where history is going to pinpoint the start of progressivism, but in my mind it started around GWB election, not 1960. Pre Progressives were just called liberals, and there used to be some democrats, now it’s just progressives and liberals.
Had you ever heard of William F. Buckley, Jr.? Your recap of American political history leads me to believe the answer is no.