What is possible is to talk about how and when folk music started to be presented as a "product,” and how songs native to Appalachia, for example, became accessible to folks in California. For this, we can thank the folklorists.
Around the turn of the century, historians became curious about how English and Irish folk songs had adapted into American culture. There were a slew of folklorists exploring, and most of them kept to a particular region of the country, or even a specific state. Their quest was to find the songs traditional to that area and to write them down simply to preserve them, since they were traditionally preserved through oral account.
This anthropological venture was taken up by several individuals and groups, though the most famous was John Lomax and later his son Alan. The Lomaxes were particularly fond of cowboy songs, while others like Howard Odum focused on African-American tunes. They and other folklorists would set up make-shift recording studios wherever they went and record the musicians they found. These "field recordings" would later become fuel for the folk revival of the 1940s and beyond.
Curiously enough, as Dick Weissman points out in his hugely informative book Which Side Are You On? (compare prices), many folklorists thought protest music to be completely separate from folk music. This would echo later during the protest song period, as friction arose within the community about whether folksingers should be commenting on current events.