bluegrass music comes from the esteemed autocratic Bill Monroe who spelled out his cultural preferences pretty clearly. It's a hierarchy that tends to be rigid and up tight. The Newgrass is far more musically interesting and is often reviled by the bluegrass "purists". Bluegrass reflects a cultural point-of-view that is marginally accepted by those not a part of that "culture".
It's interesting that there are no African-American bluegrass musicians.
That should tell you something.

Frank