That's because they have forgotten the whole point of comedy. It is not to preach to your audience and ridicule their values. It is to make them laugh.
Most late-night talk show hosts of the past got this. Jay Leno, in particular, was a master of it. I would think part of the problem is that quite a few people whom they've hired to host these things are from outside the United States. Samantha Bee is Canadian, Trever Noah is from South Africa, and John Oliver is from the UK, so it all comes off as this cabal of foreign trash-talkers putting America down. They've turned a source of humor into an opportunity to lecture, and it doesn't work.
The argument that people just don't watch their channels, I don't think that carries much weight. The channels they're on are general entertainment channels. Plenty of middle-American conservatives gladly tune into Comedy Central to see their reruns of Futurama or Archer, but will immediately tune out when Trevor Noah comes on and starts insulting them.
