Danceability was not important in distinguishing top and bottom songs until the late ’70s.
But from 1980 it became really important, maybe in relation to the rise of disco, electronic dance music and other later music.
From the late 1980s, the songs at the top became relatively harmonically more complex than songs at the bottom. Before that time, songs on the top tended to be harmonically simpler. That’s quite interesting, because somehow the opposite is true for rhythm.