Although the sixties era was supposedly the height of rebellious rock, many of the biggest stars grew up listening to the music of their parents. That means they heard mostly melodic, pop and swing music with little dissonance. And many rockers came out of the folk music scene which featured many great melodies. They learned how to sing.
Succeeding generations though heard mostly only rock, so they didn't have a grounding in melodic music.
I just can't figure out what goes for modern music today.
The wife and I have sat through numerous meals in many restaurants in the past fifteen years where modern pop/rock/hip hop was played constantly.
We are still waiting to hear our first decent melodic recording after being forced to listen to thousands over the decade.
Why can't modern musicians come up with decent melodies? It's a big mystery.