Jim,
There are several reasons why the pieces are largely anonymous. The chief one is the fact that we are talking about the very bottom of the literary market here. Half the time the printers didn't even bother to put their own names to the sheets never mind the poor fellows who sold them to them for a shilling. Another reason is that some of the writers would have been respected people in the community and some of their output as you well know was rather indelicate for the times, quite apart from the political pieces which you mention yourself.

Yes of course some of the sharper ones were simply rewriting earlier broadside songs and the percentage of alteration varied but they were still largely using the same body of material.

I'm sure you are aware that the bothy material is a very specialised , almost a one-off body of work. The farm labourers of NE
Scotland at that time were extremely isolated and were literally slaves. Several men lived in in the bothy and had little contact with the outside world. They entertained themselves in time-honoured fashion by making up songs about their harsh conditions and lampooning their employers. My own ancestors worked in a similar environment on the Wolds in East Yorkshire but were not quite as isolated and conditions were not quite as extreme. As far as I can glean they only had the one bothy song which was used universally for each farmer and this was not a broadside ballad (one of the 5%). The reason why these songs are not spread throughout Britain is that by the end of the 18thc very few places in Britain were so isolated or as poor. Notice I have not referred to literacy here. Paradoxically NE Scotland in the early 19thc enjoyed a rate of literacy among the poor well above the rest of Britain due to forward-thinking educationalists mainly in the church.

I have already conceded on several occasions that different conditions occurred both more recently and in the past in Ireland. But the tradition of composing local songs found in the 20th century in Ireland is certainly not typical of England. Whilst the making of local songs does continue in some rural areas in England thay rarely get any further than the maker.