As Kent pointed out above, it makes no sense at all to blame the Puritans for changes that evolved in the southern hill country, where there has never been any Puritan influence or even presence.

The replacement of various magical motifs with more rational storylines is probably nothing more than a general trend toward "modernization."

Why would this have occurred more thoroughly and noticably in the US than in the UK, even in relatively "undeveloped" or technologically-backward communities? Perhaps America has simply been more self-consciously "modern" or "forward-looking," even in its most rural areas (where adaptation of "modern" ways might well have been a matter of wishful thinking than of actual attainment).