It's sipmle. His main audience of 14-year old boys aren't allowed to stay up that late on a school night. That's why he had larger ratings during the school vacations in August and why it went down after school started in September.
Boy, Did you ever call that! My 14 year old son (as most all 14 year old sons who are trying to annoy their Dad will) would quote me "news" stories he'd heard on The Daily Show. The only good thing about it is that it might prepare them for the real world of broadcast news as we know it - that all of the news and all of the anchors are silly!