I think it is much simpler than this.
- Liberals take themselves too seriously to be funny.
- No one wants to hear someone tell them they are not paying enough taxes to Joe Welfare and Jane Bureaucrat.
- Everyone likes to hear about being independent, successful, and free from regulations, which is what most conservative talk show hosts (the successful ones anyway) are talking about.
- Talk radio is mainly an analytical process. Words and thoughts are the tools and logic is the rulebook. People who tend to gravitate to this and are attracted to it are conservative.
- Liberals are more feeling-oriented and tend to process information visually, and thus gravitate more to TV. If they are listening to radio they are listening to music, for the most part.
Winston Churchill said it best:
"If a man is not a liberal by the time he is 20, he doesn't have a heart. And if he is not a conservative by the time he is 40, he doesn't have a brain."