The reasoning is simple. Higher hit probability given the factors of: physical and mental exhaustion, distracting environmental effects (not just wind and shit but bullets flying your way), not always being able to have a good shooting position/stance (body armor is a fking bitch at times...), effects of recoil on follow up shots, and speed of target acquisition. While the 2nd portion of the Marine Corps' rifle qualification does include graded head shots (or pelvic shots, it's shooter's preference), it is generally preferred to drop rounds center mass and only go for a headshot if absolutely necessary. Additionally unless a headshot is placed just right, it can either skim around the skull, not hit anything vital, or be stopped by the forehead.
In a shit sandwich situation what would you prefer? Trying to get that one "perfect" shot? Or pumping multiple rounds center mass with confidence?