I've seen this question a couple of times, so I'll be brief in my answer. I encourage you to look into the roman section of the FAQ for some more in depth answers from some of our Roman experts.

In short, it looks like the increased usage of mail armour (Lorica hamata) in late antiquity as opposed to the Lorica segmentata was down to comfort and efficiency. Hamata were pretty effective - mail armour stayed around for almost a thousand years after the fall of the Roman Empire in Western Europe, and for good reason. It represents a flexible, (relatively) easily obtainable piece of armour that protects against most slashing or cutting blows. Compared to the Segmentata, which was expensive to produce, procure and maintain, it definitely had its advantages. It should be noted that the Hamata didn't 'die out' for a couple centuries. Rather the two forms of armour were used together, in addition to the Lorica Squamata, or scale armour. After the crisis of the 3rd century, Roman industrial production of armour takes a significant hit - the hamata became more common as a result.

Essentially the segmentata was only widely used for a brief segment of Roman military history, and even then it wasn't ubiquitous. Of course you won't get that impression looking at any reproductions of Roman armies on screen ;)

I'll post more sources once I'm off my phone!