I think they feel it's a betrayal of Sacred Folk Purity to be in any way favoured by the mainstream, especially as what The Unthanks do isn't exactly mainstream anyway, and yet has a wide appeal. Maybe that's another reason - I applaud anyone who might cover King Crimson and Robert Wyatt with the same exquisite attention to musical detail as Kipling / Bellamy (their cover of A Tree Song on Oak, Ash, Thorn is a definitive masterpiece) or traditional Northumbrian songs or whatever else they do. As I said on the other thread, they are an autonomous musical unit and can do as they please, which they do quite fearlessly.