Jazz died off as a mass genre for two reasons.
First, as Mark Gauvreau Judge wrote in his fun 2000 book, If It Ain’t Got That Swing, postwar economics and the rise of bebop as a counterforce in jazz greatly killed off the big bands of the 1930s and ‘40s, but the complexities of bop led many teenagers in the 1950s to seek out rock and roll as a simpler music style to dance along with.
Capitol Records putting the full force of their PR team behind The Beatles when they arrived in America in early 1964 cemented rock and roll as the dominant musical genre for teenage whites, as Nat “King” Cole, who helped make Capitol a dominant force in America in the 1950s, discovered to his horror when he called their flagship Los Angeles office that year and the receptionist answered “Capitol Records – home of The Beatles!”
It boils down to economics.
Why did rap become so popular? Because it’s cheap to produce.
I remember when Fleetwood Mac did “Tush”, I remember they spent at least a million dollars in studio time.
A rap record can be done for a fraction of the cost, and the “artists” are disposable.
I like jazz more as a niche.
People that are into it will consume everything, not just the big mainstream acts.