My guess is stereo and FM radio. Optimizing for those things in the studio made music sound different. The way people listen to music has a big impact on how it

My guess is stereo and FM radio. Optimizing for those things in the studio made music sound different. The way people listen to music has a big impact on how it is engineered to sound. I don't have the evidence to back this up, but modern popular music sounds, to my ears, very much like it is optimized for playback in headphones. IMO a lot of it doesn't sound that great when played out loud.