my guess: most people aren't "into music" (certainly not the way we are, anyway), and have no real interest in knowing what's new or hearing songs they don't know. they use the radio as background and are only interested in hearing songs they do know.
kinda like the way i use television: i have no idea what new sitcoms or cop shows have debuted this year and not really interested in spending the time/effort to learn, but i will occasionally be happy to watch a re-run episode of "law & order" or whatever.