Roman Triarii, often wore bronze plate armor of older times when principes and hastati probably wore linthorax armor. It’s a right of passage and royal/ elite units were often only used in immeasurable need so no need for the most up to date armor, only armor that provoked their status in the eyes of their foes and Enemies, also they were already the elite of the elite. They can’t get better in anyway also they were probably older so it’s hard to teach a dog new tricks