If I remember correctly during the period to which Al refers Roy Harper spent most of his time claiming that he wanted to "get off the folk page". Have a glance at the sleeve notes to the album "Back To Reality - Rot Harper 1970 - 1975". Having seen Harper perform an number of times, and I have been a follower of his since the "Sophisticted Beggar days" I would cite his anti - showbiz, nonconformist aproach to his music which while, endearing him to the likes of me, was never going to find him commercial success.
Don't blame the traditional folk clubs for that one.