Robert Johnson was primarily influential among white blues aficionados who comprised a much bigger market during and after the 50s/60s revival. When RJ's records were being primarily marketed to rural black folks, he didn't sell much. When Eric Clapton started saying Robert Johnson was godlike, THEN the record-buying public paid attention.

It becomes an interesting question: was Robert Johnson actually a great blues musician, in the sense of satisfying the aesthetic of the blues's primary community? An argument could be made that he wasn't. I wouldn't want to make that argument, though!

Leadbelly raises different issues; he was not so much a bluesman as a rural popular singer/folk singer who tackled everything from cowboy ballads to kids' songs. In that sense Woody Guthrie was very much a white counterpart. There's no shame in this; other great "bluesmen" like Mississippi John Hurt also fall into this category (MJH also did kids' songs, like "Chicken," as well as lots of gospel, though I never heard him do a cowboy ballad).