Media Life magazine had two media experts answer why Air America went south. Nancy Haynes said exactly what I've said from the beginning, that taxpayer-funded radio was the big obstacle, over 700 affiliate stations, a very established network:

Air America has had an especially tough time growing its audience because its affiliates must take listeners almost entirely from other, more-established stations, and the most likely source for Air America to “steal” audience would be from NPR affiliates. Yet NPR's isteners are among radio's most loyal. That's bad news for Air America.

Andrew Ettinger added the other obvious point, that radio dynamos like Limbaugh and Hannity built slowly, from the commercial ground up, not from the philanthropy-driven top down:

Ettinger: Yes, it can succeed. But to build a radio network you need to start from a solid base and then expand. Rush Limbaugh, Dr. Laura, Sean Hannity, etc. did not appear overnight as dominant radio personalities. Rather, they build their following one station at a time, one market at a time. By doing so they grew from strength, not artificial station clearances.

Finally, this exchange:  

MediaLife: What, if anything, can be learned from Air America's problems? Can the network recover? 

Haynes: They're toast.

Ettinger: The lesson: Go slowly. Don't expand too quickly. Let the audience grow naturally and build sales around those organic numbers, not artificially inflated ones. Of course ratings will increase as stations and markets get added, but that is not truly organic growth.