Plate armour is a product of the late medieval period.
It consisted of sheets of steel, cut and moulded into pieces that protected the major body parts from armed opponents who wanted to perforate your major internal organs. The pieces were held together either by buckles, being tied to an under jacket or riveted. Gaps between the plates were filled with chain mail. The below is the ultimate in plate development.
The weight of this mass of metal was not as great as some history books would have you believe. It is possible to run, do shoulder rolls and most importantly get up from the ground without any aid what so ever. Not a lot of fecking point in protection that turns you into a turtle on its back now is there?
Plate armour was so effective against most weapons that weapons required to evolve to defeat it. Swords changed their shape and techniques evolved to defeat armour while new weapons such as pole arms and large maces came into being to batter the buggers senseless inside their armour so you could finish them off with a dagger thrust through an armpit/groin/helmet eye slit.