I think a lot of people judge media bias by a "mainstream" that they are defining by their own environments. I have friends that live in New York City, and their idea of "mainstream" American ideas and values are far more liberal than I think is accurate. But I also have friends that live in rural areas in Kentucky and Illinois and Pennsylvania, and many of them have an idea of "mainstream" America that is far more conservative than I think is accurate of mainstream America.
So when these folks look at the media, and assess "bias", they are doing so by a criteria of "mainstream" America that is itself, biased, by the environment in which they are living. And it's understandable, too, because everyone where THEY live generally does think like they think. And believe what they believe. So from their perspective, their thoughts and beliefs ARE "mainstream". It would be somewhat difficult for them to realize that they and all their immediate neighbors are NOT necessarily mainstream Americans at all. That they may in fact be far to the right or to the left.
As someone who has lived in both the north and the south, and in big cities and in rural areas, I think the mainstream media is trying very hard not to upset anyone about anything, and as a result they are so "mainstream" they are mediocre. And almost socially/culturally meaningless. I think the media in this country does little more then act as a cultural 'echo chamber', because they have no creativity or courage at all, anymore. All they have are charts and graphs telling them exactly what most people are thinking and saying and doing and they use these to echo exactly what they think their "audiences" want to see, think, do and hear. Their only "bias" is the bias of banality. It's the bias of fear, in that they live and die by whatever audience share they can muster on any given day.