Over the years, Robert Johnson (RJ) has been the most legendary Bluesman of all or at least, one of the most legendary and famous. There is no logical explanation for it that will satisfy all probably, however, it can be possible to list a few reasons that might be some of the reasons for it and starting with the 'selling his soul to the devil' is on topic.
Almost everything known about RJ is not built on the solid legs of logic. His 'selling his soul to the devil' thing is one. Off course, one can believe it to be true. However, it is actually a legend that rooted way back before RJ's times, and even in the Blues era he wasn't the first, Tommy Johnson, another legendary and famous Delta Bluesman had the same story about him a decade before RJ. But it tends to stick better with RJ than with anyone else, although, there is no prove that RJ sold more of his soul to the devil than, say, TJ. So, I guess in case someone, who doesn't know anything about nothing, will decide to try to search and find about someone who sold his soul to the devil, he'll find RJ as the most suitable answer.
The one guy who talked about RJ selling his soul to the devil is Son House(SH). And from his words, there's no hint to know what he meant, was he serious, or maybe he used an analogy to prove something, maybe he was just talking about the sacred VS secular music, and the strong belief in his time that one singing secular music, he is singing for the devil, and one singing sacred, he is singing for God. SH also mentioned on another occasion (read the liner notes to his double-cd on Columbia 1965 sessions) that he and his mate, musical partner, and yet another legendary Bluesman on his own, Willie Brown, that the two met RJ in 1930 and that he couldn't play. And than, after a short while RJ appeared again with them, and he was playing so well they were shocked. This short period of time between RJ was a bad to average Bluesman, and the time that he had skills that were praised by his peer legendary Delta Bluesman, might be part of what make people think strongly about the devil legend. The lyrics on RJ songs are also not hurting the devil story.
Like his song 'Me and The Devil'. "Me and the devil was walking side by side, I'm going to beat my woman until I get satisfied." Now, this is aggressive and also, what does it really means? Was he acting badly because he sold his soul to the devil? Or maybe it's just a metaphor, or maybe a way to explain his two sides, the bad, that the devil represents in the metaphor, and the better one, which is represented by the word 'Me'. The 'Crossroads Blues' song also might be something that might convince people that he is singing about the night he sold his soul, but other suggestions were that as a colored man (terminology of his time) being outside when it's getting dark was his fear, because he didn't want to get caught by police or something. The same goes with other appearances of devil-related items in his music, there is nothing that really is proving or disproving the legend.
It is known that RJ used to travel a lot. Since childhood, he used to move from place to place, from his in-law father's home to his father, or maybe other family members. And after childhood he was doing the same as a Bluesman (read Johnny Shines interview about the times he and RJ were traveling from place to place), this time with his guitar and as a Bluesman. But, it was a common practice of the time. The flood of people going north to the places where industry gave them better jobs was an engine to this sort of traveling, and in the golden age of the rural Country Blues in the 1920's, many were doing it. It was less practical in the 1930's, because the Dow-Jones that collapsed ended that era and traveling musicians weren't able to make the same money as before since it, but it is not unusual still it is considered more a RJ thing than anyone else.
Going back to 1930, it is the year that possibly his wife died with his baby she was carrying. It is like a turning point, his loss, his selling his soul, than his coming up with a sudden talent that people didn't find in him before. And he was very young at the time, about 20 years old. His recordings were at age 26. But he wasn't the only one to have tragic life, nor was he the only young talented musician. But it seems to work well for his legend. And he didn't become influential or a legend before he died. Actually, Gayle Dean Wardlow (GDW), a famous Blues researcher who traveled to the area for his researches in the 1960's, found out that RJ was almost totally unknown while other musicians from the area, like the legendary Charley Patton (CP) were more known than him although CP was even of older era, he was remembered better (a quote from the following GDW interview here: "Most people had never heard of Robert Johnson, just about everyone had heard of Charlie Patton.").
In 1938 Robert Johnson died (read Chasing That Devil's Music by GDW the section about RJ), and he was probably murdered by a woman or man who was jealousy about RJ's talent or a romantic triangle. He died young, but he is not the only one. Leroy Carr (LC) died young as well, and he was very famous at the time (read 'Escaping the Delta' by Elijah Wald). At the same time, John Hammond Sr. (JHS) was trying to book RJ for his spirituals To Swing show. And he located Don Law (DL) who was the guy who recorded the two sessions of RJ. DL found out that RJ was dead two weeks before, and JHS replaced RJ with Big Bill Broonzy (BBB). DL also recalled about something weird about RJ, that when DL introduced RJ with a Mexican band of musicians, RJ played for them with his face to the wall and his back at them (read liner notes to 'King of The Delta Blues Singers' Columbia 1961). DL also claimed that RJ was a teen at the time, which was proved to be not precise with the death certificate found, but this is forgotten by the public.
Why did RJ played with his back to the Mexican band? It doesn't seem to be a nice thing. Did he have something with the devil? Or maybe, he just wanted not to expose his secrets in front of them. DL thought that he was just a shy teen, which is another possibility, and other suggestions were that he played close to the wall to create reverbs or maybe other affects, or that he wasn't paid for playing in front of that small crowd so he faced the wall, or that he was patronized with his talent over them. In 1939, about a year later, Alan Lomax (AL), a folklorist who worked for the Library of Congress (LOC), recorded and interviewed the young Muddy Waters (MW) in the Delta region. AL asked MW about Robert Johnson. It was when MW was still unknown, about 10 years before he started really making his mark on the Blues world in Chicago, a decade before MW became one of the greatest Bluesman of all time, but what made AL to ask MW about RJ? Does it have anything to do with the devil? How did he know about RJ, who was absolutely unknown at the time?
There were few Bluesmen that played a style that might be influenced by that of RJ before the post World War 2 Chicago Blues boom. And then, his influence was all over the Chicago Blues scene. Musicians learned his music, his songs, his lyrics, and used it, they were influenced by it. Why the gap? It wasn't something continuous as it was musicians going back 10 and more years back to his music and getting their influences from it. And then, in 1961 Columbia issued 'King of The Delta blues Singers' which became a hit. DL, AL and JHS were behind the reissue. Why and how did it become a hit? The same question can be asked about the 1990 'The Complete Recordings of RJ' that was selling very well. Why RJ? Did you see his photo? Is it his photo? Look at his eyes. Say what you think, it can be his health wasn't very well so half of his face looks much older than the other half, Cataract or maybe not. I don't believe the devil story. What I believe is that there is a reasonable explanation, just because we don't know it, it doesn't mean there is no such. But the spooky side of the legend is what I believe makes him very famous. And he also did some of the finest music, that too.