The missing variable here is whether the market share represented by Billboard #1 songs has increased or decreased in this time span. Is everyone listening to more homogeneous music, or is the market for #1 songs shrinking to a smaller and smaller demographic?

I honestly don't know the answer, but a few decades ago in most towns the majority of the public listened to just a few radio stations and everybody knew the top songs. In the Spotify age, I'm far from certain that's true.