A couple not necessarily related thoughts.
Worth remembering that in distant times, those who could read customarily read aloud to those who could not. For that reason, written texts have always been a possible vehicle for dissemination.
Also worth noting that hymns have often been sung to whatever familiar melody lent itself to the meter and was agreeable to the celebrants. A New York City gossip columnist once called out a well known Salvation Army leader for singing "Champagne Charlie" in Central Park, and the gentleman was forced to respond that he was singing a temperance song that had been written to what was then a very popular melody.
Finally, folk melodies are a very good vehicle for teaching music simply because they are familiar.