1) Chart toppers from previous eras are still listenable. Almost nothing on the charts today will be listenable in 2 years. Out-of-touch grandparents in the 1960s could have been convinced to listen to the Beatles. Nobody of a certain age will put up with Top 40 today.
2) It's not quite as democratic as you'd imagine. There are promotions, placements, trends, people sucking kids into clicks in big herds.
A major issue is that today music is more global, and there is a much wider, much lower common denominator.
It used to be 200M Americans, now it's 4B global listeners, Americans are a tiny fraction of that.