Armour Flexibility

[Nathan Ross]

The use of Segmentata not only coincided with, practicably, the high water-mark of the Empire when the Arms Factories were able to produce the necessary quality parts

State arms factories were only established under Diocletian. In fact, the army of the tetrarchic and Constantinian period campaigned continuously, and usually victoriously, for fifty years against every enemy Rome had and in the process restored the empire to something close to its 'high water mark'. And they (probably) didn't use segmentata!

But as you say, the change was probably connected with fighting style, and also possibly with the increased mobility of legions and vexillations - if you're away from home base for months or years on end, you want a type of armour that's easy to repair on the move...

Depictions of centurions, when armoured at all, show mail or scale rather than segmentata - this suggests that those able to afford it chose mail, and it was therefore the better quality and more desirable armour.

As Dan says, the state arms factories of the later empire may have found means the mass-produce mail far more cheaply. (Actually, I've wondered before whether these fabricae may have been able to mass-produce a cheap form of the muscle cuirass as well, and this was the 'munitions armour' of the later empire, replacing segmentata... but that's straying off the topic!)