leneeia beat me to the main point.
Q: Why do folk songs last so long?
A: Maybe they don't. We just don't know the ones that didn't.


Also, maybe they're more like yogurt than like cottage cheese.   Yogurt doesn't "spoil" nearly as quickly because it's still alive.   Maybe that means folk music is full of active cultures (transforming itself daily). Maybe it means it's crawling with microbes. Take your pick. ;)