I think broadside trade and collectors' motives are red herrings. The late collecting is the main factor. I was collecting in Yorkshire in the 60s and I can't remember collecting any of the classic ballads other than fragments of the most common ones. For instance just about every farm labourer in the East Riding could sing a variant of 'An Acre of Land' which derives from the Elfin Knight, but that's about it.

Anderson, Meredith et al would have been thrilled to have come across a Child ballad, they just weren't there! I met Hugh Anderson when he came across to Blighty and he was very interested in those ballads that had come from Britain.