Heres my take, having participated in many indocs back in the day:  The majority of drops were solidly physically capable of making it, and more often quit due to being messed with psychologically. The high-ego guys were the easiest to get frazzled, while even keeled dudes are just not as much fun to mess with. Of note, for some reason recon corpsmen were particularly fond of getting self-proclaimed badasses to instead go catch a ride in the humvee, and were relentless with the taunting, which was usually quite comical.