It was because protective armor had to keep pace with the evolution of weapons of attack. Combat was the defining feature of the noble class of medieval Europe. When nations themselves were not at war, lords were often fighting among themselves, or they were practising the martial arts through jousts and tourneys.
When maces, capable of crushing bones, began to come into use and when, around 1340, arrows shot from powerful longbows proved able to pierce coats of mail, it became necessary to develop better protection. So metal plates were designed to fit over the most vulnerable parts of the body: shoulders, elbows, knees, etc. By the beginning of the 15th century the process of encasement in armor was complete. The art of the armourer reached its pinnacle in the middle of that century, and the century that followed saw increasing elaboration of decorative armor.
Another factor in the development of plate armor was the rapid development of firearms. The first cannon fired in Europe was in Italy in 1330, at the same time that plate armor began to appear. Portable firearms appeared in armories in the 16th century. They gradually superseded bows, swords and pikes (long lances some 12 feet in length) as infantry weapons. By mid-century even the modern cavalryman carried two or three pistols and an harquebus; cavalry now combined speed and firepower. However, cavalry no longer dominated battles as medieval knights had done - the infantry had recaptured the dominant role.
Plate armor proved itself strong enough to protect the life of the wearer. Made from solid metal, it could resist blows from the new weaponry. But the same characteristic made the material difficult to work - a newly-made suit of armor which had successfully undergone proofs of strength would receive a stamp indicating its quality and was more expensive than unproven armor.
It was important that plate armor not obstruct the movements of the wearer. Plate armor was therefore designed as an ensemble of pieces - numbering as many as 200! - attached to the padded body of the soldier. The parts were all articulated to allow movement, much like the tail of a lobster. Armorers needed a knowledge of human anatomy to design armor that would move smoothly with the movements of the body.
The parts of the warrior's body most at risk to life-threatening injury were those the most heavily protected: head, chest, and shoulders. Next highest priority was given to body parts active during combat: arms, hands, elbows, knees and legs. Finally, feet were given protection. The amount of protection each soldier had depended on what he could afford and his military rank.
The king wore full armor, artistically decorated; he probably owned
several suits for different types of occasions. The aristocratic
cavalryman, such as officers, was typically well-protected by a complete,
or almost complete, suit of armor. The foot-soldier would be issued with
half armor, protecting only his chest and head; and these items were
mass-produced, not custom-designed for each soldier. The least enviable
role in a battle was the part of these soldiers.
Full body armor was, however, doomed to extinction. The reason: firearms. At first these lacked power and portability. It took a century before their projectiles were able to penetrate plate armor. To make the plate thicker offered no solution, since this just made cavalrymen heavier and therefore slower and less mobile.