Maybe people are tired of cynical, sarcastic humor. How many times can one say a knowing, cynical "oh, yeah" and and that is somehow original and funny? Maybe cynicism has burned itself out? I have flipped the dial past TV sitcoms and it seems most of the humor involves the characters insulting each other and calling each other juvenile names. I think we have reached the point where what humor there is is quite ill, and wit is nowhere to be found.
Untempered cynicism most definitively. The cynic has to be subject to negative consequences.
If you've ever listened to Jack Benny's old radio show, he'd always come out on the short end of whatever he had instigated, usually coming full circle by the same means, as an example.
The closest Letterman got to that, that I know of, was Penn & Teller dumping a couple of thousand cockroaches on him.