I don't think it's entirely about innate traits. There's a lot of training that occurs before going to some these schools. For example, in the Army you're not eligible to 'try out' for Special Forces until you're an E4 grade (not sure about officers), which may take 1.5 - 3 years to achieve, assuming you enlisted as an E1. Even then a lot of guys spend 2-4 months in special training programs before going and some don't pass until their second or third attempt.