While a lot has to do with Letterman's brand of humor—or his well-documented spat with Sarah Palin—PR is also playing a role. Though CBS declined to comment on its PR strategy around Letterman, the network seems to be leveraging the changes at other networks and piling on the celebrities, according to several articles about the show.
"Executives also have expressed hope that Mr. Letterman could take advantage of the changeover at NBC's Tonight Show, with Conan O'Brien replacing the longtime ratings leader, Jay Leno," The New York Times reported. While all the shows are keen to reach young adult viewers, Letterman is also looking to woo that demographic, where O'Brien usually leads.
Meanwhile, in a recent article, ABC News discussed celebrity endorsements and how certain stars prefer to support certain late night show hosts over others. Even in late-night TV, celeb endorsements play a role.