The question is quite simple: there is a whole hyper appreciation of the Roman Empire in detriment of the Middle Ages. This also ends when we talk about armor. Well then, it is true that the Roman armor was usually superior (of better quality) than medieval ones? Mail armor were more accessible in the times Roman Empire than in it was "Age of Mail" of 11th-12th centuries?

Why lorica segmentata disappeared but hamata and squamata kept up as armor among the romans and the germanic barbarians? According to the forum's article itself, segmentata was easier, faster and cheaper to do (and tecnically plates are better than mail). Therefore there is no reason to mail become the standard armor after the empire's crisis

Hamata can cover more of the body like the armpits and thighs. Mail is easier to repair and has some self cleaning properties which adds longevity to it. It also much more forgiving to large variety or size because of how flexible it is. It can be hastily repaired with twisted wire. Thus is better suited for standardized equipment for large bodies of soldiers than lorica segemata. Also, performance wise, if you haven't developed the rivets and clothing technology and the skilled labor based to produce perfectly fitting limb armor, trying to wear poorly attached and fitted plate can actually burden a shoulder. If you are stuck between something that protects you move in the area it can cover and something that doesn't protect you as well but covers more of your body I think it would be wiser to go with the latter because your opponent with have easier time working around the 1st armour to deliver a lethal strike. Thus there was many good reasons mail and scale survived and segementa didn't.