I think after 200 a.D the Roman's began slowly loosing their military edge to barbarians and didn't make any real advancements in armor wich could have helped them to remain dominant and survive,

Teleological history is a lie. The "Crisis of the Third Century" was a prolonged period of civic upheaval, rebellion and civil war within the Roman Empire exacerbated by a series of plagues and invasions, and contributing to recession, logistical collapse and mass inflation within the Imperial economy. Indeed, in a fifty year period, there were over 25 claimants to the Imperial throne, and at one point, three rival 'Empires' were fighting for legitimacy. What this meant was that, in practice, armies were raised, fielded and, often, destroyed at a rate which far outpaced the Roman industrial ability to supply them fully with armour, or even helmets. The military reforms carried by Diocletian at the end of anarchy, further enlarged the army and kept them on the frontier, beyond even the state's ability to regularly pay its troops.

As the arrears due to the army continued to grow, the Empire became increasingly reliant on foederati; "barbarian" tribes - usually Germanic - which were allowed to settle wholesale the frontier regions of the Empire (many of which had been largely depopulated in the wars, famine and raiding of the previous century) in return for providing their military for the Empire. In these contexts, the lorica segmentata was a largely obsolete relic which was no longer logistically viable or culturally useful to the "Roman" army.

Hey there,

Just to let you know, your question is fine, and we're letting it stand. However, you should be aware that questions framed as 'Why didn't X do Y' relatively often don't get an answer that meets our standards (in our experience as moderators). There are a few reasons for this. Firstly, it often can be difficult to prove the counterfactual: historians know much more about what happened than what might have happened. Secondly, 'why didn't X do Y' questions are sometimes phrased in an ahistorical way. It's worth remembering that people in the past couldn't see into the future, and they generally didn't have all the information we now have about their situations; things that look obvious now didn't necessarily look that way at the time.

If you end up not getting a response after a day or two, consider asking a new question focusing instead on why what happened did happen (rather than why what didn't happen didn't happen) - this kind of question is more likely to get a response in our experience. Hope this helps!