Ever since Fox came on to the scene in the US, it has grown in leaps and bounds in viewership. I don't think its because the Fox viewers are all so stupid to think that is truly unbiased, but they watch it because Fox is tilted towards the right, unlike the perceived and real left-wing biases of CBS, NBC, ABC and CNN. Even average American viewers understand all that.
CNN and Fox may be very huge in terms of viewership, but that doesn't automatically create a responsibility for them to be totally free of their biases. On the other hand, maybe their large viewership is BECAUSE these are biased and the viewers like it. :)