The biggest drawback of wearing heavy armor is the hindrance it poses to tactical maneuvers, such as the most basic running, lying down, and squatting. In short, all movements are greatly affected by the heavy armor.
Thus, protection comes at the cost of sacrificing agility. Gao Guang can no longer lie down because it would be very difficult for him to stand up again, so naturally, he has to give up shooting from a prone position, and it's also inconvenient for him to kneel on one knee, so he has to give up shooting from a kneeling position as well.
