1) Rush Limbaugh pioneered the political/talk format and has a proven track record. Others on the right have a more easy time getting a show because the concept has already been proven. Sponsers do not like the idea of going into the "unknown" sponsering a liberal talk show. It is seen as more risky to the sponsors.
2) Republicans in general are older with more money and that fits the talk radio demographic more. Liberals are either super rich Hollywood types or are young with less money which is not good for sponsors or even potential sponsors. Older people who own a home in the sub burbs are a lot better potential customer than poor college kids. If you notice the advertisments on these shows you will see it is mostly for mortgages (something the younger liberals usually can not afford) investments and "old people" products.
3) Like Rush Limbaugh Fox news produces a lot of right wingers who have good name recognition and thus would be a "natural" to have a radio show. If not for the right wing Fox news Sean Hanitty would not exist. PBS is a liberal television station but does not produce too many talk show hosts and not too many people watch that anyways. PBS is not "slick" enough for todays viewer.
4) Liberals can be percieved (even if not true!) as anti business and pro higher taxes and there are not a lot of sponsors who like that!
5) The current administration is Republican and it is more easy to have a talk show host who agrees with the currnet administration than not.
6) Liberals have way too many slogans and a lot of people are tired of said slogans. These slogans are seen as just that slogans and nothing more. "No blood for oil" "Impeach Bush" etc. Just saying the same old tired slogans over and over again does not make for good radio.
7) Liberal talent is lacking. The typical liberal who might be qualified for a talk show can make a hell of a lot more money with less work by working in hollywood. If some one is a good actor say he or she could make millions shooting a movie for a few months as opposed to working daily at a talk show. Also, liberals tend to host late night talk shows. Leno, Letterman, Jimmy Kimel and Conan Obrian are all liberal. They make millions just goofing off on tv why bother with a "serious" show?
8) Local stations have their right wing programing esablished and just run sydacated right wing programing all day long. For liberals in most cases they would have to form a whole new station with all new programing which is not easy to do. The owners of the local stations are in it ONLY to make money and so this would be the old saying if it's not broke don't fix it. If a persons owns a local station that is doing well based on right wing programing then there is no incentive to change anything.
9) Liberals are concentrated in certain geographic areas. The west coast and the north east thus making it hard to have a national show. Most people in Kansas do not want to listen to left wingers! lol
10) Liberals have gotten a bad name even though not all of them are so. A lot of times liberals are thought of as moon bats, stupid hippies etc.