My theory of what happened in the US North after WWI would be that the young people were able to buy records for themselves and early swing appealed to them as dance music, which was a socially acceptable and effective way to court! At least in Chicago, in the early '20's,the Austin High Gang was learning to play jazz as soon as they get instruments and were playing for dances at the Columbus Park pavilion as soon as they learned the songs!
The older folks stuck more with vaudeville, but the young folks were hip as soon as they could wind up the old Victrola.
Chris