Dick,
Who said Marrowbones and Cunning Cobbler are not ballads?
Apart from Jim's perfectly good explanation Child included it because of its ancient pedigree, its relationship to other similar ballads and its foreign relations. Marrowbones and Cunning Cobbler are relatively recent broadside ballads and would have been outside of Child's remit and most likely his ken.

Had Child the knowledge of Bramble Briar we have today he might well have included it on the grounds, like Hind Horn, it is based on an ancient folk tale. However there are reasonable grounds to suggest that the Bocaccio Isabella story on which is undoubtedly based was put into verse by a broadside hack c1750. Unfortunately the original has not survived or has not yet surfaced. I put on Mustrad the details of what we currently know of this ballad.

Volunteering, are you?
Brian, funny you should ask that, but I'd rather do it as part of a team.