It's because they're nuts, and I mean certifiable.
John Harris and Jim Vande Hei of Politico.com try to explain the anti-McCain media bias. They begin by saying that there really isn't any. They tell us that the first reason is that the McCain campaign sucks while Barry's soars. They tell us that journalists are professionals who ignore their personal biases and report the news. They tell us of "McCain backlash," brought about because McCain is mean to the media and has been more conservative than fun-maverick of late. They blame in on "[j]ournalists’ hair-trigger racial sensitivity." They blame it on a "bias in favor of momentum."
Finally, they admit that they've been duped by Obama:
In addition, Obama has benefited from his ability to minimize internal drama and maximize secrecy — and thus to starve feed the press’ bias for palace intrigue.
All told, their conclusion is that the media is psychologically unfit to report the news, but they are not ideologically biased.