Because when a red shirt is teleported down to the planet he rarely comes back (in fact almost never in STTOS unless his name is Scotty)
Ah! I get it. I guess I was putting too much thought into it.
Saw the reboot a couple of years ago with a fellow scci-fi friend. At the scene where Kirk, Sulu, and the other guy were skydiving onto the platform, I said to my friend “which one is wearing red?” His laughter annoyed a few people I think.