The reason? Their core audience already has NPR, CNN, CBS, ABC, NBC, etc., etc...
Good logic, but there's one very important flaw: The existing liberal mouthpiece networks don't acknowledge that they're liberal. In fact, they occasionally try to deny it. Frankin and Co. are trumpeting that they're radio for the Left. I think that, even if it's really bad, millions of loony libs will listen just to show solidarity.
Sponsor reaction may hurt local affiliates, but don't forget:
Air America is funded in part by our friends at the DNC!