The reason I argue an invisible level is that the regions where a distinct song tradition survives coincide with regions where the social disruption of the Industrial Revolution was relatively minor, and so inversely we can surmise that where it was significant, the social disruption destroyed the local song tradition. At the same time, the propinquity of the parlour piano tapped into an earlier tradition of vernacular musicality predating that, and so there is at least some vestigial heritage there. Equally, the Revivals also established a common musicality across the country which sometimes tapped into the folk tradition (carols and the West Gallery in particular) - RVW's circles simply continued that. That, however, is the root of something we may in the future come to regret, as it's the same as the National Songbook, which created a somewhat fabulated national heritage. But even that tapped into the remains of a heritage, even if only of the sort portrayed by Pepys recorder lessons, the milieu created in the Restoration by Playford and Gay.
The spread of the broadsheets is probably responsible for the distribution