I really believe that there are some fundamental mindset differences between people who are generally attracted to conservative philosophies and people generally attracted to more progressive philosophies (I know, "duh"!). It seems that those attracted to more conservative philosophies are more entertained and attracted to the gut/emotional-level cheerleading and rhetoric that is the bread and butter of successful talk radio, than those attracted to more progressive philosophies. Of course, this isn't a blanket truism, I know there are emotional/gut liberals just as there are contemplative, esoteric conservatives, but in general, and in my experience, it seems to be the case. In the most simplistic terms, I guess I'd divide the extreme ends of this scale into the navel-gazers and the fans/cheerleaders.
This may be one of the main reasons why Liberal talk radio that tries to emulate the "shock-jock" inflammatory style of most successful Conservative talk radio shows, dooms itself to limited success or failure. Again, I'm sure these perceptions are largely guided by my personal experiences.