When "Air America" tried to launch a left wing talk source, it didn't work out. I think there are reasons for this that has nothing to do with the talent of Limbaugh.
             Limbaugh came along at the same time as the "car phone," and as I remember it, most of his callers were from people stuck in traffic with car phones. It grew from there, but it's mainly a commuter thing. People in houses probably have a television on, so they aren't listening to the radio.
             I suspect if there was a way to analyze it, it would boil down to demographic differences. People who commute to work in cars being more conservative that people on sub-ways or at home, or folks who simply listen to music.
             The other sucessful talk show people are mostly right wing as well - Hannity, O'Reilly, Savage, and etc.