This is partly because of the prejudice of early collectors who saw folklore as the property of the illiterate. New England was not typically regarded as a target-rich environment for folklore collectors because it was perceived as white and educated.

When it came to folk music, John Lomax (1867-1948) and his son Alan Lomax (1915-2002) were particularly influential. They looked to the South and the Appalachians as the best places to find American folk songs, and because they dominated the field of American folk music collection, they shaped perception. For want of this sort of aggressive collecting, we don't know much about early folk music elsewhere. Other regions may have been less rich when it came to folk music, but much was certainly lost and mostly, there is simply not enough information when compared to the South and the Appalachians.

Edit: I should have mentioned that modern folklorists understand that everyone - including the literate - have folklore. The earlier perspective that only the illiterate possessed folklore was an expression of elitism.