After the Roman empire crumbled, the ancient world all of a sudden got a whole heck of a lot smaller. People were used to trade flowing in from distant, exotic places and the roads the Romans built and maintained became rundown and beset by bandits. Although the Roman empire dissolved, armor in the dark ages and later armor in medieval times continued to be developed and improved. You probably realized this because you don't see people running around in Roman Legionnaire armor, unless you happen to stumble into one of the historical reenactment events.
The period after the fall of the Roman empire is dubbed the dark ages in Europe, keep in mind this only pertains to Europe because Asia and the Middle East were actually entering a sort of golden age at this time. This period of time is referred to the dark ages because the human race resorted to huddling around campfires figuratively. Imagine for a second the neighborhood in your area. The streets are well lit, the gas station is selling snacks and useful products and is nicely lit, you see the occasional person but they don't seem overly frightening. Now imagine the street lights in your area slowly dimming until they go out and no one replaces them. The gas station gets boarded up and the people you do see on the pot hole filled roads seem dark and menacing. Eventually the only light that works is the one on your porch and possibly the neighbor next door. If this did happen to your neighborhood you'd do exactly what the people from the dark ages did. Stay inside and avoid any door to door salesmen. Most of European societies gravitated towards small kingdoms where the peasants farmed the land in exchange for protection from the lords. This set the stage for how middle age armor would progress.
Over time the people in Europe moved on and the shock of the Roman empire crashing and burning slowly faded from memory. The small villages protected by small forts grew into large villages protected by large castles. The kings in those castles soon noticed they had neighbors also in large castles that seemed ominously close. The world had once again lightened up and thus mankind entered the medieval times or the middle ages. As we all know from books and fairy tales, this was a scary time. Dragons ravaging the wheat fields, ogres stealing maidens, and trolls living under bridges waiting for billy goats to cross. Seriously though, knights of medieval times were most likely fighting crusades or the random war their king sent them off to, then rescuing maidens in distress.
As the art of refining armor in medieval times was perfected knights became protected by an ever increasing amount of metal and plates. Often the full suits of medieval knight armor are thought to be ridiculously heavy but in actuality it weighed less then the amount of equipment modern day soldiers carry.
As territory lines became more defined and kings and queens from different countries intermarried wars became less frequent. To keep the peace kings oftentimes staged elaborate tournaments. The tournaments served a number of functions besides peacekeeping too. The jousting and melee battles kept the knights in shape and the possibility of death and injury served to entertain the local populace the same way the Colosseum had served Rome. The tournaments also drew large crowds bolstering the local economies. Special tournament armor was crafted to protect the knights involved. Blacksmiths generally made the armor thicker and more durable. Helmets that might weigh 5 lbs. on a regular battlefield topped 40 lbs. in arena sets.
So what exactly stopped the dominance of knights in the middle ages? Well the arms/armor race had never stopped and weapons had again outpaced the defenses people could craft. These new weapons, as you probably guessed it, were guns. At first primitive fire arms were rarely dangerous except maybe to the person using them. When they did work properly they were weak and inaccurate and took a massive amount of time to reload. As time went by they were improved and the amount of armor required to stop a bullet was just too heavy for a person to bear. Thus a slow decline of armor began until the 1900's. For the better part of 300 years armor was again shrunk to the helmet, greaves, breastplate model of ancient Greek times.