I think the reason is that leftist politics cant survive the critical analysis and the spirited open dialogue promoted by talk radio.
NPRs closed intellectual version has an audience, but it has to be both highbrow and closed to unedited listener comments. If its too emotionally charged, its unappealing. If its opened up for comments from people like you and me, its showed to be logically flawed. Al Franken cant do anything more than energize the Democrat base for the election.
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.