Not a singular phenomenon: When people think of german folk music, they will think of bavarian stuff, not frisian, hessian, saxon etc.

As for american music the author states that the music of the southern borderlands is predominantly latin-american. Might that be the answer why it is not dominating? Not that latin american music is in any way inferior, but it did not develop in the US and is more representative for Latin America than the US.

Claiming latin american music as us-american would also be seen as cultural appropriation - the author is opening a can of worms here...