Dan Howard tells us that the main reason for adopting the Lorica Segmentata was that it was far cheaper to produce than Hamata. Furthermore, because of the wide coverage provided by a scutum, the most common area of injury for a legionnaire would be the shoulders. Lorica Segmentatas' reinforced shoulder plates make it seem as if it was developed with that in mind.

Vegetius tells us that the main reason for dropping the Lorica Segmentata was because it was too heavy. Supposedly, the legionnaires got soft and couldn't bear to wear it anymore. Vegetius lamented this slothful attitude, because of the Lorica Segmentata's greater protection in comparison to the armor of Late-Antiquity.

However, this is probably not the only reason, nor the main reason for the abandoning of such armor. This comment by u/bitparity tells us that the later emperors required a lighter, more mobile army attached to the emperor(s) that could more quickly respond to internal and external foes throughout the empire and its borders.

Lorica Segmentata also required much more effort to maintain compared to the Lorica Hamata. Rust was a very big problem for the plates. The use of leather under metal in high mobility situations would also lead to quick degradation of the leather, as it would constantly rub up against it during any sort of movement. As the empire's logistics collapsed, it is reasonable to believe that the legions could no longer adequately maintain their platemails and went with the sturdier alternative.

Returning to Dan Howard, he claims that chainmail was actually the preferable alternative in most aspects aside from blunt trauma. It allegedly provided better coveragewith greater mobility, while not requiring the legionnaire to wear additional inner lining. It also was far easier to repair, as you could use a wire of metal.

Lorica segmentata was designed largely as a form of scale/plate hybrid armor that had overlapping strips of metal that allowed a wide range of protection. Its initial advantages, namely being more sturdy and resilient as a form of armor, were outweighed by its cost, and the technical proficiency necessary to create it.

By no means was the Lorica segmentata standard-issue for all Roman legions. Only a handful at a time possessed them, and largely based on seniority. By contrast, lorica hamata, or chainmail, was tried and true, and had been used since antiquity. Everyone knew how to create and use it, and it was relatively cheap and effective.

The period in which it was created and implemented was also the high-watermark period of the Roman Empire, when the Empire possessed a technological proficiency, with economies of scale, and labor markets that would not be rivaled until the industrial revolution. What ended it largely was the period of instability known as the Crisis of the Third Century, in which the Roman Empire was torn apart by civil war and strife that lasted several decades and nearly ended it.

The resulting reconstituted Empire stabilized in the Fourth Century under the reign of Constantine and his sons. Constantine reformed the military away from a legionary model towards a rapid response model, firstly to eliminate the political threat the army posed, but also to posture the Empire towards defending its borderlands. However, by the end of the Fourth Century and the beginning of the fifth century, Rome was confronted with a new stream of problems: barbarian incursions.

While barbarians had been encountered quite frequently, and many even made deep incursions in the Empire, only to be drawn back, this was the first time in which Barbarians had scored several crushing victories over the Romans, notably at Adrianople, where the Eastern Roman Emperor was slain by roving Goths.

I second this question in regards to the use of the scutum along with shields in general in the middle ages. The use of such armor by the romans, in company with their manipold/other formations, seemed to me atleast, to be extremely effective tactics, especially against lesser armored foes. Chainmail especially seems like it would be a much more ineffective and cumbersome alternative to the segmenta/plate armor. What was the main reason behind what seems to me to be de-evolution in tactics/weapons/armor during the middle ages?