The reasons for this should be obvious. There is no need for a separate venue for liberal or "progressive" ideas and discussion, most MSM newspapers and television news already provide it. Conservative talk radio exists to provide conservative ideas and discussion in markets where it otherwise doesn't exist or is a beleaguered cable minority (Fox News).
Translate the thoughts in the above paragraph into listenership, which is what drives radio programming as it attracts advertising dollars. Conservative talk radio draws listeners which draws advertisers. Liberal talk radio has few listeners, therefore few advertising dollars, and isn't an economically viable proposition. In radio markets rule.
The article does make one valid point, it is likely more conservatives have jobs where listening to radio while working is a reasonable thing to do.
