The Air America "farther left" experience scared owners deeply and lastingly.
(And we forget that we have some great centrist-moderate liberal content on NPR that out-rates local conservative talk in most markets)
Air America did work in a couple of markets, most notably Seattle and Portland. But it faded after a year or so.
My evaluation was that the hosts were crusaders, not entertainers. Rush was fun to listen to... Al Franken, Randi Rhodes, David Bender, Thom Hartmann and the rest were more "campaigners" than entertainers and their intensity and pushy advocacy of their beliefs got tiring.
A good evaluation point is Rachael Maddow who does well on TV, where she has a shorter show and lots of producers, guests and scripted content assistance. But "on her own" with three hours of radio to fill every day, she ran out of content, tended to be repetitive and did not have all the great guests she has now.