Progressives are more likely to be listening to more modern types of conversation and distribution methods, like podcasts. This was already true when Air America launched.
That doesn't mean they don't particularly listen to the radio, but I doubt you'll find many, especially in a market like San Francisco, a technology hub scanning the AM band for talk radio.
Air America did OK in Madison for awhile. On FM.
As far as why Conservative Talk is more successful, it's not only because that audience is probably more likely to be listening to AM, but the hosts are good at keeping an audience engaged. Everything is horrible, everything the left is doing is an outrage, how dare they, the host is mad, you should be mad, the listener is so mad and they want to know more, the host is on their side, the host has more to say about this terrible thing they are doing, more on the way. The listener stays tuned. They have a firm, generally loud voice and are able to keep the listeners attention and keep them tuned in by literally pissing them off. If anybody on the left is listening, they too are probably pissed off by what they heard and call BS on what is being said, but they might just stay tuned to see what else the host has to say. They cannot believe they just said this. On and on it goes.
Progressive Talk in general has been less like this. It was generally a bit more cheery, some anger in there too, but they don't have that firm constant anger the Conservative Talk side has. They don't have as much of a hold on their audience. Stephanie Miller is one of the longest running Progressive Talk hosts in syndication. There's a lot of jokes and sound effects in that show. It's not a full time rage fest, and therefore people might be more comfortable with tuning to another station. Sometimes people on the right also call in and argue on these stations.