Superstition is one thing, but mysticism is somewhat different. I wonder if it is not so much secularism, as posited by Joe, or puritanism, but rather protestantism, where mysticism is a pretty rare attribute.   There is nothing mystical about the protestant Devil. He's simply a bad-ass. And while spirits and the daemons in the Old Country may have been frightening, I don't think they were always regarded as "the evil dominions of Satan."      Descended from ancient cultures that preceded the spread of Christianity, it seems there was (and is) a view that 'old magic' supernatural simply does not come entirely under the dominion of the Church.   

Also, think of all the legends and mythologies of the many different cultures and tribes who settled and conquered the British Isles in waves going back into prehistory, and how they became woven into the rites, beliefs and ceremonies of the Catholic church, and persisted, passed down through generations. They were also often often deeply associated with the land or specific areas, especially the mythical creatures.   There are little hidden places, and larger ones too, in the British Isles associated with the supernatural, the origins of which go back probably a thousand years or more. When the people of the British Isles moved across the ocean, they left their faeries, elves and leprechauns in the Old Country, for the most part, and didn't find new species of those fey beings here. Since they did not assimilate, in any way, with the native populations here, and since the cultures and mythologies of native Americans were so different, they also did not adopt or adapt, for the most part, any of the mystical or mythical elements of Native American mythologies or spiritualism.

New World immigrants no longer had the the daily reinforcement of their long cultural histories around them in force. Families were broken up, the keepers of the stories - the purveyors of these older traditions who passed them in not only in story and song, but in daily practice that socialized another generation to them, often did not make the journey across the ocean.    And if they did, think of the difference in the power and sense of realism between, "When I was just a lad, I saw the faeries dancing one night right by the spring hidden by Hadrian's Wall at the edge of the village. Be careful you stay away from there at night, Child", and "When I was lad in Scotland, I saw the faeries...."

The culture(s) of the British Isles did not cross the waters intact. In New World protestantism, the supernatural and magic were (are) nearly always regarded as the work of the Devil, that big Bad-Ass guy trying to trip you up. And the Devil is nearly always viewed as a trickster. And if you lose to the Devil, it's your own damn fault for not being 'good.'

So, being as the folk process involves, as others have said above, change, and usually change to reflect the meanings and values of whoever the current 'folk' are, it is not surprising that a good bit of the magical element has gone from the American derivatives of those old ballads.

Which is all an awful lot for some one to say who really knows nothing at all about the subject:>)

Having said all that, let me confess I don't know a thing about any of this.