Charles Hamm discusses this at some length in his book Yesterdays: Popular Song in America.

I can't remember the details, but the shift is associated with a period of radical change in the music industry. In the 19th Century there were major writers and publishers in many US cities. But in the early 20th Century New York and Tin Pan Alley suddenly became very dominant. Hamm also suggests that whereas popular songwriters had previously been a fairly diverse bunch, those of the early 20th Century were drawn from a fairly narrow (mostly Jewish) milieu.

There is no connection to country music, and Hamm is also fairly scathing about the extent to which the music of black Americans really influenced the popular music of the time.