I think it’s just the medium. AM radio is a very old medium with, as one might expect, a very unusual demographic.
Add to that the fact that AM talk radio has been so heavily right wing for more than 20 years, that most all of the remaining audience that even bothers to tune it in anymore isn’t looking for liberal commentary.
Liberal commentary also doesn’t take to radio very well. It does better in print because of the more complex, nuanced thought needed to appreciate it and carry it off. This is not to say that complex conservative thought doesn’t exist. However, it doesn’t take to radio well, either.
If you want complex, higher-level conservative thought, do what the left does; turn to print. National Review, Weekly Standard.