Or maybe, in their culture, knees are the ultimate erogenous zone and must be kept covered at all times, provoking interesting reactions when they run into humans in shorts.
Or maybe their home planet is infested with ankle-biting critters that can't jump (which could be an in-game hazard for opposing forces).
Or maybe they evolved in an arboreal environment, and their legs aren't strong enough to carry their full weight (+1 when moving through forest or woods).
Or maybe it's just fashion (no in-game effect, unless they are fighting Gok Wan).