[watersb]

Armor is hard. It slows you down, and you can't hear or see as well as the guy coming up behind you.

If you are trained well enough to be effective with metal armor, you're probably trained well enough to be effective without it in close quarters.

Heavy plate requires mounted cavalry, but largely negates the maneuverability of horse, and can be brought down with foot trained with pikes or heavy spears.

Maybe the metal was best used elsewhere.

[hermitcrab]

Medieval warriors who could afford it wore plate armour, because it was (despite some drawbacks) incredibly effective protection. Many modern tropes (they had to be be winched on to horses, they couldn't get up if they fell over etc) are just plain wrong. The medieval warriors were less educated than us, but they weren't stupid.