I blame two issues. Firstly, the emergence of cable news networks in the 80's. They'd get "stuck" on a major story and, from there, the temptation to editorialize to keep viewership was inevitable and the lines between "news" and "opinion" got obfuscated. Secondly, the repeal of the "Fairness Doctrine" that required news outlets give equal time to the opposing point of view. This paved the way for news outlets to "polarize".
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The later emergence of the WWW just made it worse.