WHY MEDIA BIAS?
On Tech Central Station, Tyler Cowen makes a good argument that the media are biased, but not for the reason you think:
Both left-wing and right-wing commentators lament media bias. The right wing cites the predominant Democratic orientations - often 80 to 90 percent - of major journalists. The left wing cites the right wing pundits, such as Rush Limbaugh, or the growing success of Fox News.
Why do the major media sometimes slant to the left, and other times slant to the right? The answer is simple: viewers want them to. We look to the media for entertainment, drama, and titillation before objectivity. Journalists, to get ahead, must produce marketable stories with some kind of emotional slant, which typically will have broader political implications. The result: it looks like media bias when in fact journalists, operating in a highly competitive environment, are simply doing their best to attract an audience.
I think there's also some self-selection bias on the supply side. Particularly opinionated people--Paul Krugman and Bill O'Reilly are pictured on the TCS site--are attracted to opinion journalism. People suspicious of institutions are attracted to investigative journalism. And people who want to hit it rich go into other fields.
But Tyler's point is absolutely true: You may hate certain types of commentary or certain types of stories, but generally they're wildly popular with someone. Read his story here.