Of course, even this can't be simple.
The folk music tradition of the Appalachian mountains is all bound up with race and class in complicated ways that are under explored and under researched.
It didn't just *happen*, it was born out of British, Scottish, and Irish folk traditions. But it isn't just a continuation of those traditions.
It started by melding those things together and it was blended and merged with various African folk traditions and native American folk traditions and south and central American folk traditions and Spanish folk traditions, largely as a result of the huge amounts of violence the British and the Spanish inflicted on Africa and the Americas.
And what came out of all of that was American music. The blues, Country, Bluegrass, Folk, Western music, etc. etc.
I've read a lot about this stuff, I've studied it because I want to understand it.
There's no good summary here, and no simple narrative. Lots of different people brought lots of different musical ideas together, and a lot of the examples that we have to study of this (that is to say the only stuff that survives) is pretty horrifically racist and that makes studying it really difficult, emotionally.