From what I've read there's a fair amount of debt about the prevalence of the lorica segmenta (the second armor pictured) and for much of the Roman period mail was the dominate type of armor. Add to that the fact that the lorica segmenta was expensive to produce and mail is actually very effective as far as armor goes. The first breastplate looks like a bronze breastplate, which where replaced by more durable & stronger mail armor.