Actually, I don't think that's it at all.
Fox is constantly crowing about how they're killing CNN and MSNBC in the ratings, but the truth is that, in the grand scheme of things, no one watches Fox, either. Not compared to American Idol or Dancing with the Stars, or how many people saw the latest blockbuster movie or whatever.
I should preface this by saying that I don't watch enough of any cable news to call myself an expert. Now and then I'm tempted -- but that's why I have MLB extra innings.
From what I
have seen, Fox is clearly different in that they're pro-active. Led by their editorial folks, they try to get out in front of the news. MSNBC is much more reactionary, in contrast. They respond to what Fox puts out there. They spend their evenings throwing <s>red meat</s> Cliff bars to their core audience, but they don't really try to get out in front on issues.
Maybe their lower ratings make that more difficult, but I don't think that's the full explanation. MSNBC, for better or worse, isn't that ambitious. Are they still slanted? H*ll yes, of course.