The younger demographic is more important to advertisers, the older demographic is more desired by networks; older people rely on television for their news far more than young people, who instead get their info online (which is why liberal radio is such a failure).
I like O'Reilly even though I disagree with him on most stuff. He's an asshole sure, but so is Matthews but I like him as well. See, they know they're assholes and aren't afraid to admit it, while Olbermann seems obsessed with being a pretentious ass.