Up until 1967, much of the popular music you heard on the radio was of the Top 40 variety. AM stations ruled and if their parent company owned an FM station, the AM programming was often just simulcast on the FM frequency. But 50 years ago, something happened to change all that and the course of music.
In 1966, the Federal Communications Commission ruled that as of January 1, 1967, radio stations in cities with populations of 100,000 or more had to offer different programming on their FM stations. No more simulcasting.
With nothing to lose and no audiences to speak of, a lot of station owners turned their FM signals over to people who wanted to play music that was outside the Top 40. This was the beginning of progressive rock radio.
When people discovered it, they almost never went back to AM Top 40. Record labels then started feeding these new outlets with new music. The radio we have today is a descendant of that government ruling in the US in 1966.