I believe that it is the result of the Grateful Dead and their fans taping their shows. It makes the music available, able to transcend time, stay alive. Their unique playing style also helps, as you must try to listen to every tape. If you get into it, you must be a student of the music. Hence, I wide array of Deadheads, some of which weren’t even born until after Jerry died. I don’t see Elvis pulling that off.,