Not one of the ballads in Child's collection came through any Traveller source
Nobody knows where all Child's ballads came from - he was largely working from print despite his despising the broadsides

And none of his print sources mention Travellers at all.


We know that in the twentieth century they were a major source in Scotland

They were a minor source. By an enormous margin, most of what people sing or play now has never been transmitted through them.


There is no reason why that shouldn't always have been the case

There is not a flicker of reason to believe that. Travellers were even more marginal in past eras, with less opportunity to communicate with the rest of society.

The fad for Traveller culture (hyped up by Hamish Henderson and MacColl/Seeger) is now long past in the folk scene. There are only two active British performers I can think of who still exploit it, Thomas McCarthy and Sam Lee. Nobody in Scotland does.