Think back to the 1960s-1980s...media bias didn't really exist. Journalists were journalists and everyone wanted the news as it was, not opinion. The concern was more whether or not media was government propaganda over journalistic credibility. That was mostly because of the Vietnam War and the public losing favor of it though.
What changed? Satellite/cable booming in the 1990s. "News" became an entertainment product. Radio followed suite because there was big money in political opinion syndication. This is what started the trickle of money away from professional journalist towards opinion and focused reporting. Higher ranks meant more advertising dollars and entertainment is all about the money.
Then 2000s, the internet became mainstream. Professional journalists now had to compete with amateur bloggers. A battle they could not win.
Then 2010s, the straw that broke the camels back: smart phones and social media--everyone is an amateur journalists. There's no longer money available to do much if any investigative journalism. The professional journalists are always the last on the scene. And advertisers want opinion over hard news.
Money fundamentally caused the collapse of professional journalism...it left the industry. Even historic greats like Washington Post which was responsible for exposing Wategate are chaffing funds today.
In my estimation, the media losing professional journalism came first and the advertisers drove them to bias. The bias created echo chambers that draw people and the echo chambers themselves caused the divisive political climate.
The journalists themselves are living in the echo chamber so they're incapable of being unbiased. There was an editor that quit New York Times recently that said as much.
The "beginning" was somewhere in the early-mid 1990s.
Leftist:
a member of the political Left or a person sympathetic to its views. aka
left-wing politics