They/we looked for folk songs - simple as that, the fact that Sharp defined pretty clearly what he meant by folk songs excuses him from not inclding all the other stuff that was circulating at the time
Harker's "may well" is indicative that it is his assumption and his alone that the collectors own tastes affected what was collected - as he never discussed the songs or the singers he had no grounds for making such a claim
Hoist on his own petard - I think
Later work has proved beyond a shred of doubt that the singers Identified with and took ownership of the songs they and as being 'local' or 'Norfolk' or Scots.... (or wherever they might have originated) tends to vindicate Sharp's decision as being pretty accurate
Jim Carroll