From The New York Times:
The current tendency to political polarization in news reporting is a consequence of changes not in underlying political opinions but in costs, specifically the falling costs of new entrants. The rise of the conservative Fox News Channel caused CNN to shift to the left. CNN was going to lose many of its conservative viewers to Fox anyway, so it made sense to increase its appeal to its remaining viewers by catering more assiduously to their political preferences.
So why do people consume news and opinion? In part it is to learn of facts that bear directly and immediately on their lives – hence the greater attention paid to local than to national and international news. They also want to be entertained, and they find scandals, violence, crime, the foibles of celebrities and the antics of the powerful all mightily entertaining. And they want to be confirmed in their beliefs by seeing them echoed and elaborated by more articulate, authoritative and prestigious voices. So they accept, and many relish, a partisan press. Forty-three percent of the respondents in the poll by the Annenberg Public Policy Center thought it ”a good thing if some news organizations have a decidedly political point of view in their coverage of the news.”