record labels produce their own compilations for radio stations, tv, clubs, and other place where there is a lot of people. Most of the times Each record label releases many different cds one per music genere. Most of these releases are monthly and include 20 songs. And of course these songs are priorities to the label. Other songs are sent into Full Album or CD Singles... and in the end it is up to the radio music director to choose music.
And if you are independent you can go knock on the radio stations door and ask for an appointment with the music director or programmer and give him/her your CD...