The answer's simple, WhatsUp.
Liberal talk radio doesn't succeed because talk radio appeals to folks looking for an alternative voice, not the same ol' stuff they get everywhere else.
The mainstream establishment media is overwhelmingly liberal. Whether it's the New York Times, the Minneapolis Star Tribune, the Washington Post, the Boston Globe, NPR, NBC, ABC, CBS, or CNN, they are all liberal. That voice is out there and easy to find.
Talk radio, and to a lesser extent the blog sites, has fostered an audience by highlighting alternative voices that don't reach people any other way. Thus, the popularity of conservative voices in talk radio and on Fox News (although Fox is really not as conservative as people believe... they hire a couple more token conservatives than the other 'nets, but their headlines still pretty much follow the NYT status quo. What Fox IS, is somewhat balanced as opposed to always leaning left.
Also, Air America was a political, not a commercial, effort, which is why it failed.
It was built from the ground up to help liberals win elections, not garner Arbitron ratings.
By contrast, Rush, Hannity and others in talk radio have succeeded by being ratings successes, keeping affiliates happy, drawing advertisers, etc. They were built to achieve commercial success, not just to influence elections.
