"The way I heard it was not that the Delta was the only source of the blues, but that it was the first source that caught the attention of mainstream culture, when rural singers from the Delta started moving into New Orleans in large numbers to work on the docks."

The familiar myth that blues moved from the Yazoo-Mississippi Delta to New Orleans at some point was apparently based on the preexisting myth that we ought to think blues was in the Delta particularly early relative to e.g. Louisiana, for no reason anyone can actually give.