Because, god knows, we wouldn't want our soliders to actually have effective body armor. Cuz, you know, we ENJOY sending them into situations where they're going to be shot at, and then forbiding them from taking the best body armor available. Purchased out of their own pockets for in excess of $5,000, none the less.
This Pinnacle Armor Inc Dragon Skin is good stuff. It's superior in both bullet stopping power and in flexibility to standard military issue OTV/SAPI vests. I can think of no rational reason whatsoever that the armed services would not want their troops wearing it. The only reason I can think of that someone in command might forbid wearing the stuff is that they simply do not give a damn about the lives of our troops, and are perfectly happy to see them die because they are wearing inferior body armor.