It's because they're giving the audience what it wants. Here's the ratings for Wednesday night. Anderson Cooper came close to equalling O'Reilly at 8PM and he actually whipped O'Reilly in the repeat showing later on.
And not long ago O'Reilly was beating him 2 to 1.
http://www.mediabistro.com/tvnewser/category/ratings
And not long ago O'Reilly was beating him 2 to 1.
http://www.mediabistro.com/tvnewser/category/ratings