Maybe the reason for the foreign material is repertoire envy.

There are a lot of cultures around the world with a genuine tradition of folk polyphony that goes back centuries or millennia - Georgia, the Basque Country, several regions of the Balkans, large parts of Africa, Polynesia, North America. Apart from the Gaelic psalm tradition, Scotland is not one of them. Anybody making a harmonized or polyphonic choral arrangement of a Scots song is dragging it kicking and screaming into somebody else's musical culture. Ultimately it just gets easier to bow to the inevitable and get your choral repertoire from some culture that knows how to do it right.