Clay Johnson's 2012 book, The Information Diet, discusses this phenomenon in the context of CNN going from a prime place in the news firmament to an also-ran when TV stations started driving news based on primary on their audience's political preferences. For a while, CNN still enjoyed a place as a definitive source whenever big events happened (national elections, disasters, etc.). But even that role was later subsumed by the biased TV sources.

In addition to viewer's preferences for their political slant on the news, I think that when CNN gave up the 24-hour Headline News, that probably diminished their role as well.