Adam,

I agree with you and Masato, actually, that a very interesting question is "why would the plague theory develop without evidence." The answer, I think, has to do with how folklore has been defined and studied, and (in the end) involves your second point about origins and the folklorist.

"Folklore" (originally "folk-lore") was coined as a "good Saxon compound" to replace the term "Popular Antiquities." Antiquarians were interested in studying old stuff. Early theorists of folklore held that the truly important meanings were almost never known to the holder of the lore, but had to be ferreted out by the folklorist, because scraps of lore were really the detritus of Mythological or Historical materials that had been forgotten. Early folkorists thus had a vested interest in connecting any scrap of lore they collected to Mythology or history, the older the better. Of course, it's ironic that folklorists seeking origins essentially ended up constructing "creation myths" of their own!

This attitude has long been discarded by folklorists in academia, but there is still, I think, a fascination with old historical and mythological origins among laypeople who have read some of the classic conjectural works of folklore/myth scholarship, like Graves's The White Goddess and Frazier's The Golden Bough. It is a form of interpretation that has a sturdy tradition behind it, though it is mostly discarded by professionals today. I would guess you need look no further than the enthusiastic readers of books like those to find the source of the Plague interpretation.

As to why folklorists have largely given up studying origins as such, I think it's because the kind of work that developed out of origins-seeking, Historic-Geographic analysis, came to seem both fruitless and pretty boring. After a long, extremely laborious process, the folklorist would conclude "it appears my best guess is that the story of X originated in India in the 7th Century, though I may be wrong about this." To which most younger folklorists in the 1960s began to ask "so what?"

I agree that origins and conjecture can be interesting, but in the end if I'm going to believe any origin story, I need some evidence.