I’ll pitch that this is just the natural result of repealing the fairness doctrine by Reagan in 1985.
The fairness doctrine started out in 1949 as a policy to avoid having the top broadcaster’s of the time (NBC, ABC, and CBS) create a biased public agenda by leveraging their audience monopoly. They were required to provide multiple contrasting view points on issues of public importance.
When Reagan repealed the fairness doctrine, the influence on media outlets to pitch left or right leaning view points only, took off.
There’s a certain irony behind the source info being from the Ronald Reagan Presidential Library & Museum website.