Because the liberal viewpoint can't stand up to logical arguments. Liberal talk radio depends upon a real good screener to keep logical people off the air. Eventually you wind up with the host looking like a fool on a regular basis either by being out argued or constantly hanging up on people. What you have left is either a show that is a monologue with no callers or a show that is just preaching to the choir as everyone else has gotten disgusted with the lack of interaction that should be involved with talk radio & moved on.
Brother. The Ed Schultz show doesn't screen. Nor does the Randy Rhodes show. They take on all callers. I don't think you listen to liberal talk radio enough to be able to form the opinion. I don't hear many intances of the hosts hanging up on the callers. Usually conservatives don't even bother calling. It is mostly liberals calling in to agree with the hosts. Just like in right wing talk radio.
By the way, I don't think there is anyone more structured and that utilizes screening more than Rush Limbaugh. I listened to him for at least eight years. I still tune him in from time to time to hear what he's saying. You want to talk about monologue? Open line Friday sometimes doesn't even get to the open lines in the first hour!
You want to talk about preaching to the choir, how about that Sean Hannity. (need barf smiley)
Franken doesn't lack for material. He gets on several guests each show, and never runs out of material when it comes to exposing right wing lies.
Both sides spend a lot of time preaching to their respective choirs.