I'm actually rather surprised that "Get up and bar the door" isn't more widely collected. To my mind it's an easily memorable joke with a simple storyline and a punchline that rolls off the tongue, so just from the point of view of narrative transmission, I would have thought that people who heard it would pick it up without too much trouble.

The reason I think this is empirical: basically, having heard a couple of versions of this sung at folk clubs, but not yet having learned a particular version to sing myself, I found that the story and enough of the key words had lodged in my head so easily that I was able to sing my own version to a tune of my own invention without too much hassle and without needing to look up that many of the gaps. I'm not saying that this is exactly the same process as oral transmission among the communities where these songs were collected, but just as an experiment in thinking about how easily songs can spread, I would say that I found this one much easier to pick up and relate in my own way than many other ballads that I've heard more frequently and yet still can't recall much of.

Compare it to some of the other Child Ballads with more convoluted plotlines and arcane references (surely just as arcane in the 19th and early 20th centuries), that don't lend themselves to transmission quite so easily, it seems like a prime candidate for wide circulation without too much variation - to use a technical term, it just struck me as a "stickier" song than many others.