Air America was always more of a partisan political operation, rather than a business. Explicitly built to counter the nefarious influences of Rush Limbaugh, Sean Hannity, etc., they never understood the nature of their real competition. Revenge isn't usually a viable business model.
NPR. Established pretty much everywhere, with the benefits of tax subsidies, university alliances, etc., NPR has already cornered the market for liberal leaning talk radio commentary, and for the most part, still carries a patina of a non-political operation.
The other main factor leading to Air America's demise – beyond their shady finances – if we look at the broader market for left wing commentary, it's already saturated. By the NY Times, Washington Post, CBS, ABC, NBC, CNN, etc. There just wasn't that much unmet demand for their product.