Robert Johnson may not even have known that such a thing as an electric guitar existed.

I know that jazz great Charley Christian was the earliest exponent of the electric instrument, but I'm not sure of the date he first recorded with it. His work may have become public before Robert Johnson's death.

In any event, if electric guitars were available at all in Johnson's (brief) lifetime, they were certainly still very rare, and I doubt that any were sold to poor folks in the rural south.