Writing in the current The Public Interest, William G. Mayer discusses "Why Talk Radio is Conservative" (not available at the link).
He surveys and rejects the standard explanations (the Rush Limbaughs are better show people than their liberal counterparts, there are fewer radio-sized liberal sound-bites, corporate owners block liberal talk shows, etc.). Mayer suggests, instead, that the easy answer has to do with the fact that there are simply more conservatives in America than liberals. He cites poll data on ideological self-identification that has conservatives dominate 1.8 to 1. He adds that this does not necessarily translate into electoral strength because most Americans' lack of a well developed ideology means that they can be all over the place on specific issues.
If true, it also suggests that national elections are conservatives' to loose.
He also mentions that other media are widely seen as liberal-biased, creating a demand that talk radio addresses. Also, the liberal side is more beset with fragmentation and identity politics; their minority constituencies are more likely to tune into Black or Hispanic radio than Air America -- which the WSJ reported last week has money problems.
One part of the puzzle that Mayer does not address is the following: in his of heart of hearts, I expect that Bill O' Reilly knows he is on the right ("fair and balanced" notwithstanding); most of his colleagues trumpet the fact that they are -- while I expect that most of the TV network news and national newspaper types actually see themselves as "fair and balanced" rather than the liberals that they are. The oldies come from the era of the FCC's "fairness clause" when open political bent was taboo. They may have come to believe their own posturing
The historical explanation helps to explain how the news media terrain has come to be settled the way it is. Much like the fact that the Northeastern U.S. remains heavily settled -- and the population center of gravity is only slowly moving towards the southwest -- because in 1620 the Pilgrims bumped into Plymouth Rock instead of Long Beach, California.