Fellow broadcaster Michael Medved explains on the Daily Beast site:

There’s an unbridgeable gap between the dynamics of conservative media and the imperatives of electoral politics. In order to succeed in radio, you don’t need to win a majority of Americans, or even a majority of Republicans, or even a majority of those who are listening at the specific time of your broadcast. In fact, a show that consistently commands 5 percent of the available, major market audience will earn millions and count among colleagues as a spectacular success. Limbaugh himself, who often (but not always) dominates ratings around the country, almost never scores more than 10 percent of the big market listeners who tune in to some form of radio during his three-hour daily show. The leading metro areas each boast well over 50 radio stations, so a program that draws even a mildly disproportionate share of the audience on a reliable basis becomes an attractive proposition to advertisers and to programmers.

Talk radio, in other words, appeals to a niche audience—drawing only a small fraction of the public even with its most successful shows, but still connecting with millions of people. The secret involves the fact that nearly everyone in the country listens to radio regularly—with a weekly audience most recently estimated at a staggering 242 million. This means that Limbaugh need not appeal to progressives or moderates or apolitical sports fans in order to maintain his franchise: he can remain a media powerhouse with an exclusive audience of hard-core right wingers.