I noted that many blues performers (singers/guitar players) sit during their performances. It seems to me that this is much more common in blues than in any other genre. Is there a reason for this?
I don't have a scientific answer, but I guess you could say that blues isn't exactly an adrenaline-filled music. Punk,rock, metal etc are kinds of music that makes you feel hyped and alive. They are the kind of music that makes you want to be standing and jump or run or whatever.
Blues (and Jazz) although, is the kind of laid back enjoyment. You sit on the table with a drink and relax listening to the music. The artists have to pass that vibe to you, so they have to act similarly.
Maybe because of the guitar? Classical guitarists always sit while playing, with the guitar resting on one leg. Many of the early blues players had a highly-developed technique -- very different from classical guitar technique but developed through many hours of practice nonetheless, most likely sitting down. (Think of Robert Johnson for instance.)
Generally, singers sound better, or at least louder, if they sing standing up than sitting down. They can expand the lungs farther. This, and the ability to play while strolling around, probably led most singer-guitarists to put on the strap and stand up -- and (for some of them at least) to get plenty of technique out of practicing standing up.
Most blues singers who don't play an instrument sing standing up, as far as I know.
So I guess the answer is: blues guitarists (singing or not) sit down because that's how they learned it, and rock and mariachi guitarists stand up because that's how they learned it.
Maybe it also has to do with hambone (body percussion) being done sitting down. Also, I doubt country musicians had a guitar strap.