It’s because its not funny. For example take the Bill Nye segment about water on the red planet. We had Bill Nye trying to make people understand what a monumental idea that water on Mars is a big huge great deal. Then the the rest of the panel acted like the Hyenas from Lion King. Larry, did not help, the segment dragged to a crawl of heckling and guffawing instead of working towards jokes with a point. Colbert and Stewart always made jokes with actual points. Bill Nye could have been a great straight man in the segment, but they couldn’t understand humor enough to actually use him as a straight man. I am sure everybody in the writers rooms LOVE to see their work reduced to an unearned jerk around failure.
That and who the heck wants to pay for cable anymore?