I don't think it's simply about what the market wants. If it were, you wouldn't have the majority of opinion-masquerading-as-news channels with a similar politic bent, you'd have it about down the middle, just like the consumers in the market itself.

I think the move to opinion is more about influencing decisions of the viewership than giving them what they want. Fox started it right around 9/11 as an approach to drum up support for war, and it worked, and the rest saw how effective that was and adopted it cautiously, of course the common thread and my implication is that there are agendas that they all have that this approach helps them achieve better than just delivering information would.