Liberal talk does just fine and has had the benefit of being on FM for decades. It's called NPR.
I've always wondered if Air America could've served as a launching point for a progressive talk star if it had had a better business model. Al Franken was probably more in it to start a political career, but either he or Randi Rhodes might have been able to do something if the network had displayed a little common sense. As an example, I had a friend who ran a station that carried Ed Schultz. Despite being in the conservative Ozarks of heavily rural southwest Missouri, Schultz did pretty well. He had no trouble selling out the show. He wanted to see if he could add Franken or Rhodes to his lineup. Air America said no. He had to take both if he wanted one, and he had to run both of them between 6 AM and 6 PM. The only way he could've carried both while keeping the successful parts of his lineup would've been to tape delay Rhodes to 6 PM - 9 PM. That was a deal breaker.
NPR is "moderate". Liberals perceive it as conservative and conservatives perceive it as liberal.