"Why could it not have been robust a hundred years earlier?"
It might well have been, we have o way of knowing.
"were interested in collecting such songs in the early 19th century simply because the songs were so common"
In limited areas, this might have been true, but over a century or so, there weren' actually many collected and because they were treated as artifacts (sort of like butterflies) we know virtually nothing about the backgrounds.
Even in a relatively healthy tradition (like we found in the Irish Travelling communities) - if you went looking for singers, you were directed to one or two people.
This immediate area of Ireland was rich in songs right up to the 70s, but the vast majority of them came from half-a-dozen singers (take a look at our West Clare County Library website - The Carroll Mackenzie Collection will find us).
Jim Carroll