Interesting thread. In Germany we have the same phenomenon. In my hymn book I found several old tunes of late medieval folk songs and some love madrigals of the Renaissance, thence the real hits of the time.
The best known christmas carol, written by D. Martin Luther is "Vom Himmel hoch, da komm ich her" = Down from Heaven I'm coming, bringing you new tidings. It is in style, and using its tune, formed after the usual songs of the wandering minstrels starting "Aus fernen Landen komm ich her" = I'm coming from far lands and are bringing you new tidings".
The problem arises, in my humble opinion, because you have more poets than composers, and it is more difficult to write a tune than verses. So to make popular a text it is best to use a well known tune in propagating a new hymn; especially in the times of Reformation this technique was used.
Terminus technicus is contrafacture, using a non-church tune for a hymn and vice versa(!)

Wilfried