The stereotypical Roman legionary tends to wear the lorica segmentata, a cuirass made of metal bands attached to leather straps, providing protection for the torso and shoulders but also being lightweight and, by the looks of it, collapsible. Wiki indicates its last use was in ~C4 AD, but seemingly more for political reasons due to the loss of a legionary's status. I'm also aware the Romans had scale and mail armour, which both seem to be earlier inventions.
Was this armour not great? Were the scale and mail better and the segmented a bit rubbish but looking amazing? Or was it good and the ways to manufacture it were lost? What replaced it when it did go out of use?
In the same vein, the stereotypical Roman helmet, the Galea, with or without it's horsehair plume. Not enough protection for the face?
Adrian Goldsworthy has it that the Lorica Segmentata was a small improvement on Lorica Hamata, a mail hauberk. The Segmentata was slightly lighter than mail armor and had good protection in the shoulders particularly and may have been better protection against spears and arrows. However both armors were in use at the same time in the Principate, or early portion of Rome's Imperial era.
Lorica Segmentata fell out of use because it was harder to maintain. It's bronze fittings caused corrosion with the iron plates, and the fittings and straps often broke, which we know because the fittings are commonly found in legionary encampments. The complexity of the design probably required a degree of technological skill in the Roman military infrastructure to construct and maintain.
Mail armor, on the other hand is fairly straightforward to maintain. It is also easier to resize, or recycle bits of mail from one armor into another. As the empire became more decentralized, this advantage in construction and repair became more significant over time.