I think that deregulation, as is the case with many industries, led to this kind of very specific, niche identification within the news industry. When news programs were no longer required to give 'equal time', they were free to program however they wanted, and thus discover their markets.
To be clear, news events are fairly non-partisan. Events occur, and the story must be told. However, how a story is told, and what questions are asked, can vary in tone. Some people prefer stories with one tone or another, or with political views that support their own.
Wo what happens when the news media becomes a market? How does one compete? Timeliness? Done. Quality of coverage? Subjective, but no matter what, something your competitors can do. Find an audience? What do they like to be told? What kinds of stories get the most coverage?
And so, news outlets begin catering to their audiences, telling them the same news stories, but in very different ways.
One might argue that this is how things should work - the marketplace of ideas. However, from a public service perspective, this is a dismal failure. Audiences self-select into their markets and do not pay attention to opposing views. Some people prefer chunky peanut butter over creamy peanut butter, and it's hard to get them to even bother trying what they don't like. The same holds true in a news market.
Of course the most sensible people realize this and sample from multiple outlets. However, most people are not so sensible.