I think the rationale for making it his A version is simply the couplet form which generally speaking and in other cases tends to be older, or perceived as older, than the quatrain format. The same would appear to apply to Scandinavian ballads. I don't think how a variant had continued in tradition had any relevance to Child's ordering of them. He seems to have used different criteria at different times to his chosen ordering, sometimes what he perceived as the best/fullest copy, often Mrs Brown, came first, but at other times he orders them chronologically in order of when they first appeared.