For one, people are not consuming music in the same way. They're not hearing things on a mass-syndicated radio network and then going out and buying albums based on what they and their friends like...they're quietly adding singles to their Spotify playlists. Their playlist is unique to them. To discover new music, one has to consciously seek it out. Whereas, in the past, it was prohibitive to carry more than a few records or cassettes along with you, people now carry more music in their pocket than they can listen to in a lifetime. So the discovery problem has dramatically changed. It's also harder for young artists to strike lightning and make a significant amount of money, so I think that impacts the amount of talent entering the scene.

There is no more 'mainstream'. In an age where you could only get music on a few radio stations and what could fit in a physical record store, selection was a lot more limited.

In the 60s pretty much every teenager listened to rock. In the 70s disco came and you couldn't get away from it. 80s glam metal and rap started. All the youth watch MTV.

Now, you can listen to whatever you want to on YouTube, Spotify or Apple Music. There are tons of popular genres: edm, rap, hiphop, rock, pop, etc, etc. The reach of the biggest pop stars is much reduced. In the 60s everybody knew who the Monkeys were. I can't think of any modern manufactured band that has that name recognition. Last ones were probably NSync / Backstreet boys.