I don't know if anyone has done this definitive study, but in my opinion, the most obvious answer is because television and the newspapers are so heavily dominated by the left, radio is the only medium where our opinions ever really get a fair airing. Limbaugh came in at a time when the vacuum was so large, it was begging to be filled by someone, and boy did he big time.
I remember saying at the time Air America was starting that it was doomed to fail, because the market for liberal opinion was already pretty well saturated in the media. Even on the radio, NPR had already been around quite a while to appeal to the mainstream left. Air America is designed to appeal to the ulra far-left MoveOn.org George Soros crowd who doesn't feel that NPR is liberal enough. And that crowd is a pretty small slice of America, and virtually nonexistent outside of San Francisco and the Boston to DC Beltway corridor.
P. Diddy (for example) isn't listening to talk radio. Rosie O'Donnel isn't listening to talk radio. If they aren't listening to talk because they prefer music (or mindless trash passing itself off as music), why would they tune into talk just because it's left-wing talk.