If you look at the images of Roman legionaries and photographs of reenactors, you will notice that the armor of the ancient state does not protect the arms and legs. A fair question arises as to why, with all their advancement by ancient standards, the Romans did not think of equipping their soldiers with such important elements of equipment as leggings and bracers?
This is because, standing on the hill, the famous reformer of the Roman legions, Gaius Marius, shouted: "Do not feel sorry for the legionnaires, the Romans are still giving birth!" But seriously, the lack of protective equipment for arms and legs in the Roman legions had very specific rational reasons, dictated primarily by the tactics of fighting as part of the legion. In fact, the Romans were quite advanced and knew very well about the means of protecting the legs and arms (just look at the same Roman gladiators). And most importantly, the slave economy of the ancient state, especially by the time of Lucius Cornelius Sulla (138-78 BC), could well have provided the legions with greaves and elbow pads, if not completely, then at least in some representative number. But there was simply no need for all this.
To fully understand the question, you need to delve a little into the history of military affairs of Ancient Rome. In fact, the entire history of the legions can be divided into three large periods: the period of the use of manipulative tactics, the period of the use of the tactics of cohorts, and the period of gradual departure from the tactics of cohorts, which began at the end of the Roman state. We are interested in the first two, because the Roman army in the last period did not look at all like the brave legions well-known to everyone from the works of artistic culture.
The essence of manipulative tactics stands on two pillars: the division of the entire army into tactical units - maniples and the division of all manipulations on the basis of veterancy: gastats (young recruits, "elephants"), principles (experienced veterans, "grandfather") and triarii (the elite of the early Roman army, "Demobilization"). Although maniples were the forerunners of the cohorts, on the battlefield they acted completely differently. The early Roman army was not much different from the troops of its neighbors: the Etruscans, Carthaginians, Greeks. The Romans were also built with a continuous phalanx - a line in three rows of maniples. In front were recruits, behind them veterans, in the rear - the elite. This tactic, among other things, gave rise to the famous Roman proverb: "It came to the Triarii." Its significance should be obvious - we are talking about the fact that in some important matter everything went very badly and the positive outcome hangs literally by a thread.
It is important to understand that although the Roman phalanx was three-row and polynomial (in the form of maniples), it was still the same phalanx. Moreover, the very early legions still used the long spear as their main weapon. However, this gradually began to change. Primarily due to the fact that Rome was in a very good and at the same time unfortunate place. On the one hand, the position of the city on seven hills was very beneficial in terms of the economy, but on the other hand, it also made it a tasty morsel for all its neighbors. The Romans were forced to fight for protection, but at the same time they perfectly understood their position and had serious ambitions in the field of expansion. The bottom line is that they had to fight a lot.
And once you fight a lot, you inevitably lose a lot of people. The worst thing for any army is the veterans. The main trouble with manipulative tactics was what was the trouble with any other phalanx. They demanded a sufficiently high quality of fighters in terms of physical and drill training. Each new generation of recruits had to learn for a long time, and when you are in a state of war, there may simply not be time for that. As a result, the average quality of the army sags, and with it successes on the battlefield sag. Interestingly, just in the era of the Roman phalanx, legionnaires perfectly used greaves (they usually wore one on the leading left leg, put forward in formation) and elbow pads. However, this era was receding into the past.
Due to the impossibility of regularly and quickly training all recruits to the desired level of quality, the Romans began to give more and more preference to remote combat - throwing darts. A gradual shift from the use of long spears in favor of several throwing short ones began. This is because to throw a dart you no longer need to be as strong and enduring as to handle a long spear. Demanding on the rigor of holding the line is also reduced.
And then gradually the Romans reached the tactics of cohorts, which finally took shape during the time of Sulla and the reforms of Guy Maria (157-86 BC). The Roman manipulative phalanx is finally a thing of the past. It was replaced by cohorts, which were no longer necessarily built on the field in one line. Gone is the three-member division of cohorts according to the principle of veterancy. Now there were just legionnaires. The cohorts included both recruits and veterans, as well as Evocats (legionnaires who retired, but then returned to serve on a contract). And all this became possible due to the fact that the main weapon of the legionnaire was not a sword or a spear, but a throwing dart - we saw. And at the same time, the need for close combat has disappeared.
Of course, the legionnaires fought hand-to-hand anyway. However, most of the battles were by no means the kind of carnage that modern people are accustomed to seeing in films. Moreover, the cohorts in every possible way helped the enemy "make the right decision" by throwing darts in his direction. Nevertheless, it is important to understand that the hand-to-hand combat of the late legionnaires was already far from the same consistency, frequency and ferocity as the hand-to-hand combat of the early legionnaires, who, in their logic (form of application), differed little from the same Greek hoplites.
Hence the answer to the main question posed at the beginning: there was simply no urgent need for the protective elements of arms and legs for most soldiers. And therefore the ubiquity of their use is a thing of the past. At the same time, in many antique images, greaves and elbow pads can often be seen at the tactical commanders of the legion - centurions and Roman standard-bearers - aquilifers. Moreover, certain legionnaires from among the most worried about their health probably spent the money they earned not on beer and prostitutes, but on the purchase of additional items of equipment. Although the Roman legion was a big step for humanity on the way to creating a regular army (and in many ways it was), it was still an ancient army. Therefore, there was no exceptionally strict regulation on the form of clothing in it, and such amateur performance was not prohibited.
Continuing the topic, you can read about how they lived gladiators in ancient Rome: whether they had a chance to survive in the arena.