When all stations had was single-day experiences in Hooper and Pulse and then, one week snapshots in Arbitron, we did not realize that "favorite station" is not the way radio listening works.
When we got the PPM, we realized that the average listener has two or three or even four primary stations. They listen more to one this week, and maybe more to another next week. They don't "stop listening" to any of the favorites... they just go through cycles or moods of listening a bit more to one than to the others, and that changes over time.
Further, many people have some additional choices they use occasionally. Maybe for a game, for breaking news, for occasional party music, or for traffic reports or the like (still some use that). So listening is a blend of total radio time spent listening using different ingredients each time.
Oh, and music stations change over time, mostly based on what regular music research says to add, slow down, speed up or drop. And talk stations vary widely based on topics that are sometimes hotter than other times. But stations don't change for the sake of change... they are constantly refreshing the playlist, imaging, and other content items.