With immigration from so many places, American traditions are a melting pot of many countries and cultures. What floats to the top is quite arbitrary. If you re-start history at some point, say, Hitler dies, Stalin dies, Versailles is different, etc., these events should have no impact on music. But simply from the butterfly effect, the forms of music that form the mainstream can be different. What happened as recording technology suddenly modernized in the fifties and sixties would define the dominant elements of composition. Very much comes from the contributions of a few people like Chuck Berry, Buddy Holly, Elvis Presley. Any slight variation changes the direction of music in that period. Then there is the jump to Britain and back. After all, if Glenn Miller survived, he would be 70 in 1974.