Not until the 20th century was "English literature" (still less "American"!) thought to be a subject worthy of university study.

Classical and Medieval literature (in the source languages, of course) the chief topics of philological interest. The Pepys and Roxburgh broadsides , for example, were scrutinized and published (and referred to) by only a small number of people with antiquarian interests. They were generally thought to be worthless as literature.

"Literary theory" as an academic discipline with contending aesthetic and sociological positions did not exist. Intellectual belief was that whatever was of value in literature written in Modern English was readily accessible to any intelligent reader.

Aesthetics was a matter of established taste that had been formed by the rigorous study of Homer, Vergil, Cicero, and other Classical figures. In the academic world, the free verse of Whitman, for example, was widely regarded as doggerel.

Seen in that intellectual context, Child's decision to devote much of his career to the cross-cultural literary study of ESPB was arguably unique and obviously trail-blazing. (It is certainly possible that he was drawn to the subject partly because of his own working-class origins.)

As a reminder to Mudcat: Child wrote a substantial article expressing much of his mid-career thinking about the ballads, which appeared (as "Ballad Poetry") in Johnson's New Universal Cyclopaedia in 1874.

It's too bad he didn't update it for his five-volume collection.