From Tristram P. Coffin, The British Traditional Ballad In North America (1950), p. 17:

Rationalization is one of the most powerful of all the forces that work on ballads. In Britain and America as belief in ghosts, fairies, and other spiritual characters dwindles, everyday substitutes are provided, so that an elfin knight becomes a gypsy lover and later an illicit lover or even the lodger, while a mermaid is replaced by a mortal, if mysterious, sweetheart. So strong is such rationalization that most of our modern versions of the old ghost, witch, etc. ballads have lost all or nearly all traces of the supernatural. Thus James Harris generally appears today as a triangle love tale between three mortals, the harp motif has nearly vanished from The Twa Sisters, and Sir Hugh's body seldom speaks miraculously from the well. Of course, certain ballads are still completely retained in their supernatural form, but these are usually out and out ghost stories or religious tales like The Suffolk Miracle or The Cherry Tree Carol that would not survive if rationalized. But, on the whole, the devil, the elf, the mermaid, and the like have left or are leaving the songs. Barry's explanation of the Croodlin Doo evolution of Lord Randal demonstrates the trend.