The Center for American Progress has released a valuable analysis of the factors that account for the huge ideological imbalance in political talk radio. Here's what they pinpoint as the driving forces:

Our conclusion is that the gap between conservative and progressive talk radio is the result of multiple structural problems in the U.S. regulatory system, particularly the complete breakdown of the public trustee concept of broadcast, the elimination of clear public interest requirements for broadcasting, and the relaxation of ownership rules including the requirement of local participation in management. Ownership diversity is perhaps the single most important variable contributing to the structural imbalance based on the data. Quantitative analysis conducted by Free Press of all 10,506 licensed commercial radio stations reveals that stations owned by women, minorities, or local owners are statistically less likely to air conservative hosts or shows. In contrast, stations controlled by group owners--those with stations in multiple markets or more than three stations in a single market--were statistically more likely to air conservative talk. Furthermore, markets that aired both conservative and progressive programming were statistically less concentrated than the markets that aired only one type of programming and were more likely to be the markets that had female- and minority-owned stations.