Back when the charts were diverse only because there was great music of diverse types at the time. Brit Invasion, Garage/Psych, Motown, Pop, Bubblegum, even Country crossed over often.
We were so lucky to be young and having all that great music coming from our radio speakers...
true, radio stations nowadays just don’t have that diversity in their formats.
And some stations, which advertise as being oldies stations, such as playing 70s 80s 90s, have very small Playlists. If you listen to a station for a few days, you will hear the same songs over and over.
I read that some such stations have playlists of about 600 songs. But if you look at old record charts, you will see that thousands of songs made the charts in those years. For whatever reason, programmers ignore the vast majority of popular songs, including some which were huge #1 hits.