Shadow guidex
The roman plates were made of two types of material, mostly. One was bronze, relatively easy enough to make a mold and shape it through it.
the other was the Lorica Segmentata, which was strips of metal attached to leather straps.
The roman thecnology for armor was indeed very superior to the armor from the dark ages, but the crafting of it was lost.
nevertheless, even then, only commanders and special units had such armor, they did not equip 2000 legionares with it.
Also, if you guys remember Ad&d and mayber even the Ad&d equipment guide, you ll see that both the bronze breastplate and the Lorica segmentata were inferior kinds of armor to the Field Plate and the Full Plate
The techniques for the full plate and the superior grooved gothic fullplate were only developed at the very end of the middle ages, (right about the same time when the techniques for making greatswords were also developed), at that time, fire arms became more and more common, and armor became largely obsolete and too restrictive.
Making Articulated Pieces of Metal that effectively cover Most of the parts of the human body While allowing it flexibility and movement, Was VERY hard to do. The techniques developed for fullplates were researched and used to make Space suits for NASA, because they couldnt figure it all out themselves.
Leather was indeed much easier to find, because you could somewhat mass produce it, making several armors at the same time and it didnt need full days of work for months at a time on one piece of armor (you just had to wait for the chemicals to act for great part of the production time.) That and the fact that it was A Lot easier to learn how to make it, and also, a lot easier to fit different sizes of people with the use of leather straps and such....
Edit addition:
My group at the present likes to travel light and they are adventuring in a very hostile environment (cold mountains) they dont seem as interested in scavenging bulky pieces of hobgobling made armor to bring back to the poor mining community and try to sell it to the Rock Gnome merchant who is a trader of Dwarvish weapons and armor. That, and the fact that most superior magical equipment they will get they will have to find, or make themselves, since there arent magic item traders roaming around the dangerous contrisides with enough valuable equipment to buy an entire city block.