The Trio network used to show old episodes of Letterman when it was on the air. Then the network died and reruns of old Letterman shows never were picked up by any other network.
Why?
It seems it would be great on a network like Comedy Central. Surely they could bump an hour block of reruns of Tosh.o or some other shows to make room for Letterman reruns!
There are a couple of reasons
The show was in standard def, so it looks dated (yes, I know there is METV and COZI and others out there running old shows)
The monologues are usually based on the news; jokes about old news doesn't play as well as when it was topical
Many guests were plugging the movie/TV show they were doing at the time
I definitely like some of the old episodes and I can see why purists would want to see more of that stuff, but I don't know how appealing it is to general audiences.
Well Dave (in the early days of Late Night) relied much less on topical driven monologues than Johnny Carson did. Much of Dave's early humor was based on zany antics (making bad phone calls, throwing stuff off a five story tower, wearing a suit made of Velcro and sticking to a wall, remote segments, etc) or theme shows, and didn't rely that much on current events. And although Dave did have guests who came on the show to plug movies and TV shows, he also had a recurring lineup of more offbeat guests (Brother Theodore, sex therapist Dr. Ruth Westheimer, Kamarr the "discount" magician, Sandra Bernhard) who generally weren't plugging anything.
The good news is that Antenna TV has been running complete episodes of Johnny Carson's Tonight Show for the last few months, so I'm holding out hope that they might add Letterman to the lineup too.