Probably because it was collected and preserved by "intellectuals": the Library of Congress, the Child books, and so on. Then in the '50s and '60s it was popular with young people, many of whom were in college -- and the US has long had a love/hate relation with higher education. The whole thing is ironic because folk music came originally from the people themselves.

Bluegrass is just one of the variants of the music played by the folks, but it doesn't have "intellectual" and "education" splashed all over it.