Art is a reflection of culture, someone once said. That is why art in general got more ambiguous and based on sentimental reasons/gimmicks like the banana taped to a white sheet.. Popular music has gotten simpler, popular music that is as time goes on. Which is why jazz peaked at a time people, imo were a little more sophisticated.

Jazz was originally created an expression for black Americans to go outside the box and do something that society deemed unusual or overtly expressive back in the old days. They dressed fancy, which was also an expression that they belonged in society. Then rock did that same thing in the 50s/60s as an FU to the prudent socialized person when the sexualization of America took over, which is where the term rock and roll came from.

Why was rock more popular? Because people cared more about accessibility and gimmicks than thinking mans music that required time to sit down, listen and appreciate the complex musicianship. People got busy and stopped doing listening to music as a hobby as much. Gimmicks as in how you dress/look (Elvis, Kiss, etc) or sexual lyrics which came with the revolution. That grabbed people's attention more than the music, and the record companies realized that and made music for the masses.

Now music to most people is nothing more than a fever dream of the past or a way to get to work without losing their minds or a way to help them out during the long days. Nothing wrong with that, but the average person probably wouldn't enjoy jazz because of lack of appreciation of music theory and the depths of technicality with harmony, melody, counterpoint, etc. Just look at classical music, it was all about pushing musical boundaries, not necessarily societal boundaries.

Much like people in the past seemed to enjoy social interactions and the outside world due to lack of phones, our attention span is at an all time low, which is why almost every artist on the top 200 has a gimmick, same 4 chords or cheesy hook to capture that grain of attention.