Because people like to hear about things they've never heard before, especially if it's scandalous. The more people learn about the Van Joneses and ACORN type scandals from Fox News and don't hear it from the left wing media, the more curious they are about Fox. And the more they watch the more they like what they see.
It's also a fact that Fox is more representative of the country.
"PRINCETON, NJ -- Conservatives continue to outnumber moderates and liberals in the American populace in 2009, confirming a finding that Gallup first noted in June. Forty percent of Americans describe their political views as conservative, 36% as moderate, and 20% as liberal. This marks a shift from 2005 through 2008, when moderates were tied with conservatives as the most prevalent group."
This also helps explain why the left wing media is failing so miserably.
All of which is a good thing, a very good thing.