"But to what extent is it also due to those folk revivalists who used to believe that the only folk music worth listening to was British and white?"

Well...the world music phenomenon, which wiped the floor with British folk commercially in the 80s and 90s, was all about traditional music from other places, notably Africa and South East Asia. One of the things that you'd notice, though, at gigs or if you went to festivals like Womad, is that the audiences were still predominantly white. This is why I don't really buy this idea that folk music attracts mostly white audiences because of trends within the revival, or because of some inherent sense of "this isn't for you, it's only for us" - because programming music by black artists within genres that are deeply rooted within black traditional music does not necessarily translate to substantial black audiences, either.

I've got lots of theories about the whys and wherefores of this (because of the work I've done in culturally diverse audience development), but I won't bore you with them. :)