> the vast majority of the broadsides never made it into the folk repertoire - a bit difficult to discuss in the context of Roud's book as he bungs everything into the melting pot and calls them all 'folk'

No, he is explicitly concerned with the songs that the folk did sing.

Possibly some broadsides never sold very well. When they sold in reasonable numbers there must have been people who intended to learn the songs. Nevertheless, as we all acknowledge, most of those songs were not subsequently found by collectors. That could be because the people who intended to learn them had second thoughts. Or they sang them only briefly and then cast them aside. Or perhaps they did go on singing them but none of their friends and families took those songs up. Tastes can change a lot from one generation to the next, or even quicker. The songs that we love tend (though not exclusively) to be the ones that have endured because they have appealed to successive generations, albeit possibly only to an interested minority in each generation. That's your "selection" at work.