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It's heavy, it won't stop bullets. Ceramic plates are even heavy than steel plates, have to be replaced if you are hit, and are expensive as fuck.
EOD units have special body armor which is as close as you are going to get to a modern equivelent of full plate armor. It is slow, heavy, hot as fuck, and needs to be completely replaced if it is actually damaged.
It is simply cheaper and easier to replace soldiers then it is to outfit what already exists with impractical armor.