The reason Knights abandoned chain mail for plate armor during the Middle Ages was simple.
Have you ever seen what a crossbow can do?
It’s not pretty. It could shoot through the thin metal pieces of chain mail and straight through the leather.
So plate mail was invented to protect against the crossbow, primarily.
But as plate mail became thicker and thicker in order to withstand more and more powerful weapons, it became too bulky to be very effective in actually waging war.
It was like wearing a tank.
