QOTD : A media bias

The Economist had it's own take on media bias while writing about the Republican primaries and the near sucess of Santorum over Mitt Romney.

if anything reporters and commentators usually like to pretend they play as small a role as possible in influencing the public's choices. Rather, it's because the analysis strips away the fiction that the media are a neutral communications channel between candidates and voters, and turns attention towards the real influence that the media's natural biases—the bias towards surprise, the bias towards cliched sentimental background stories, the bias against sophistication or complexity, etc—exercise on campaigns.

 So taking the above into account what do you think are the causes of media bias?