@kbm0 @dnkboston @Tarnport
Well... actually the shiny plate armour arrived late in the middle age, close to the end.
Armours must be resistant and light (well... as light as an iron suit can be ๐). They need technical skills and tools that took time to discover and master.
At the beginning they just used thick leather covers, then moved to chain mail and only at the end they could build armours.
EDIT: Sorry... I read again your post and maybe you were saying exactly the same thing. But I'm not 100% sure so I'll leave my post.