I suspect that sound quality may also be an issue. The quality of many of the pre-war L of C blues recordings was appalling, but there's enough interest in blues to guarantee minority specialist interest sales for even the most poorly recorded material. The same cannot be said for mountainy singers like old Bascom unfortunately.

Don't forget of course, there's the Smithsonian Folkways CD, Ballads, Banjo Tunes and Sacred Songs. SF CD 40082
Also, that fabulous ten incher, Smoky Mountain Ballads. Folkways FA 2040. Long since deleted, but downloadable from the Global Smithsonian site.