Short answer: Trade secrets. Look at the catalog of the player. A guy who spent his life making rock music is a rock guitarist. A guy who spent his life making blues music is a blues guitarist.
If I call a song and tell you "this one is a flat tire shuffle" would you understand what I'm talking about? Are you familiar with 8 bar blues? 16 bar blues? Can you recognize the difference between a texas shuffle, a chicago shuffle, and a rock shuffle? How is your 12/8 slow blues? If I say "this one has a quick change", do you know what I'm talking about? If I call a dozen blues standards will the average rock guitarist be familiar with those tunes? Not usually....
Most rock guys seem to be fixated on minor pentatonics and think that's qualification enough but in blues the major pentatonic plays a much larger role and most rock guitarists do not have the same grasp on it. Playing the changes is more important in a blues setting.
If you get booked for a blues fest, show up and play white room and purple haze you aren't likely gonna get a return invite. Granted, some blues fests have fallen to the "anything goes" mentality but most of the fests down in the south are not going to put up with that.
IME a great rock guitarist will be able to stick a real blues gig but the average rock guitarist will fall apart by song number 4, especially when I press him with four or more choruses of soloing. Guys like Hendrix and Clapton are very adept at blues but they spent their careers releasing rock and pop records, not blues. There are rock guys whose blues playing has my deep respect and have had an oversized impact on blues guitar but they are still rock guys at the end of the day.