It has nothing to do with ratings.
Stern before he went to satellite was runing about 4 or 5 million listeners. Rush has about 15 million, Hanity has about 13 million, many conservative talk show hosts had better ratings than Stern.
A lot more people know Larry King's name but his audience is about 1/3 of O'Reilly's.
I worked in radio for a few years. Here is my two cents, 1) Stern at his peak had about 12 million in his audience.
A far cry from where he is now.
2) The difference between Stern radio stations and Rush's radio stations is that you have to pay Stern (CBS) to put him on your station. Rush is free to have, you don't have to pay him to have him on your station.
3) Morning radio is the most competive when it comes to ratings as opposed to afternoon and drive time in the evening.
4) Stern wasn't on that many radio stations across the country as opposed to Rush who is in all 50 states.