It is the real or perceived bias that drives the right leaning viewers to Foxnews. If you work under the assumption that there is in fact a liberal bias on the other major news networks, it makes perfectly good sense that the one conservative leaning network would outperform the others as it's core audience is not split among multiple channels. To steal a line from Boondocks "the absence of evidence is not the evidence of absence".
It is also a legitimately arguable point that the more left leaning portion of the American population probably does not watch political talk shows. Those people in a racially and socially profiled vacuum are the core audience of Honey Boo Boo, South Park, and the Real Housewives of New Jersey.