Leather Cuirass Lorica Musculata, I used to think no way but

[Magnus]

Honestly Magnus I can only offer opinions as to why but then we’d debate our opinions.

That's true...such discussions are usually better in person, and it's hard to drink beer through a monitor.

The fact is the sculptures do appear to show a stiffened but flexible garment. Whenever the wearer is slightly cantered or bent the garment always bends with them; metal does not bend like that.

No doubt...I know exactly what you are saying...but to me this can be easily explained by the ideal represented in the sculptures of the perfect body. In my mind, it's not an actual representation of an individual, rather a god-like personification. And not just emperors, but high ranking officers as well. So the bendy-armour to me is simply an artistic convention showing off a perfect body, but at the same time demonstrating the ornate nature of musculata worn by that class. Since it's art, it can kill two birds with one stone.

And consider this...opinion or not but you know what cracked, creased and stressed leather looks like. If it is indeed leather...why didn't the artist show those creases in the statuary? So much other detail is often added, why not depict that as well?

Also, wearing leather, regardless of how ornate or even painted it is, the amount of time and skill in it's crafting wouldn't compare to one made of metal. So if it's status and social standing you are after, which are you going to wear?

The officer’s knot always appears to pull on the garment and the garment usually appears to have a leather lace up at least one side, with a tied knot at the top under the arm pit. There is no aesthetic reason for the artist to add a leather lace with a bow tie under the arm pit. Many show leather hinges as pointed out by Travis, not metal hinges. We never see the pins on Roman Cuirasses worn by the Emperors holding the front and back together. Unlike the metal Greek cuirasses which have pins the Romans are laced.

Can you answer why that is?

Better than Jim's answer? Not really...but non-metalic materias have been used to tie body armour together for centuries...from teh afore mentioned segmentata, to using silk lace to tie japanese cuirasses together.