One of the most important positions at a radio station is the Program Director. One of their jobs is to select the right DJ's for the stations format. They are selected primarily for their personalities, and not necessarily for any in-depth knowledge of the music format. The most listened to times of day are mornings and early evenings. Commercial radio stations derive most of their revenue from selling air time to advertisers and sponsors.
Different formats attract different listeners.
For example, Alternative Rock stations attract the 12-35 demographic, and in turn there are companies and products that look for that segment of the population to reach with their ad messages. Whatever the format or segment of the population is selected, the stations goal is to choose music that appeals to the broadest possible audience within a segment of the population. Record labels select what specific songs will be played on commercial radio stations. Payola is rampant in the music business of the 1990's. Payola has been around in one form or another since the 1950's.