Guys no one suit of armour is the same! If you actually try wearing plate armour and compare it with ringmail you can find and test things for yourself! ;D
Chainmail and lammelar is the more flexible. It is very strong and will stop most slashes and cuts.Forms of iron and steel lammelar , scale and chainmail far outdate any european iron/steel plate armour.The romans and greeks actually did use plate armour however that was made of bronze which has a lower melting poiunt and is easier to cast in large pieces like cuirass/breastplates etc. the reason why steel/iron plate armour came so late is becuase it was too expensive and the technology to cast large pieces of iron was not ready.
Also by the dark ages the roman scientific method of warfare like "ducas impetuas punctas im mortalis"(a two inch stab in the right place is fatal!
was duly replaced by the gothic and tribal art of cut and slash.Since most medieval armies are made up of men who prefer to slash than stab mail was the better choice as armour. In Euroupean culture the early frankish knights as well as the saxons before them had perfected the shieldwall formation with mailed warriors. Chainmail isn't just worn alone as some would think.....
A true warrior of the time would dress himself first in a thick aketon or gambesson. This could be made of several materials like wool or heavy linen. However the aketon itself was thich enough to stop most close rance cuts and slashes, now image that padded garment with chainmail on top! ;D The first crusade was a classical example of this effective weapon. if you look at most of the battles actually fort in that campaign most occured in autumn season or like doralyaem took place in colder mountainous terrain. The aketon mail combo was perfect however the longer summer battles forced many knights to remove the aketons due to heat! then as you can imagine they were fine against slashes which both arab and turks were rather good fond of ;D however against composite bows they were not as effective.
Now if we look at the composite bow (something all steppe nomads are particularly fond of ;D) as you can guess it was originally a hunting weapon and later often used in defensive and aggressive battles. You will notice that although the bow remained pretty much unchanged for four milenia (OK so some examples did use string stretchers :
you cant help but spot the huge variety of arrows. Early war arrows were no different to the hunting arrows and these were fine against the animals and unarmed men but against armoured soldiers they were not exactly that good. Thus many were refined to the point that i now have several iron war heads that can punch through modern kevlar and dragonskin! Iron being the principal material used in the arrow head becuase it was cheaper. However i have obsidian example which are even lighter than iron and have a greater penetration. Obsidian is a stone! The average war bow had anything from 70-150lbs of power required to draw thew string. The closest weapon the europeans had to that was the crossbow! While the much publicised longbow is also very good it has one drawback!
That is it is a simple self bow of wood only and thus prone to breakage when fully drawn to 100lbs. The composite bow made of horn, wood and sinew (often having no wood) was much more powerfull. Longbows are also d-bows i;e they when strung make a typical Dshape. Composite bows are recurve meaning that they actually curve away from the archer when unstrung! Therefore they have a higher potential energy even before the arrow is notched!
Plate armour can stand up to many slashes and cuts as well as thrusts. True it can even stop several arrow types, but remember in a battle an archer can shoot at least 6 shots a minute with no problem (10 seconds to load and fire is actually quite slow i can manage it in 2 or 3). Now with an arrow storm plate armour can stop the odd arrow here or there but massed volley fire will result in at least one arrow finding its mark! ;D Plate armour is also obviously more rigid and therfore harder to move in. Now you work it out a slow moving armoured knight even mounted will be under fire from five hundred yards by the time he has closed in to 100 yards he will be a human pincussion! If we say that arrows did not penetrate deep enough to kill him the fact that some were often splayed with poison or just plain dirty or rusted even a small cut would be serious trouble!
The only weakness in the archer is his vulnerability to close quarter combat. Horse archers on the other hand can always make a swift gettaway! ;D The other problem archers might face is soldieres with shields. an average 28 inch arrow will be definatley fatal to an armoured soldier even if only half of it passed into the victim half of 28 inchese is still too deep a wound to be easily dealt with. However most arrows will lose thier impetous once they pass through even a half inch wooden shield, so although a soldier could seriosly injure his arm while holding a shield, his body as long as he holds the shield a few inches away from it will be spared the full impact!
Now those armour fans that are still shaking thier heads at me can just take a look at agincourt as a perfect example. At agincourt a disputed number of english archers no more than 10,000 strong faced a massed french army of about 30,000 of the finest plate armoured noblemen and knights of france. The french ccharged at the english lines with much vigour however the english warrior archers had planted stakes to stop the horses headlong charge and poured volley after volley of arrows at the french with thier much simpler longbows, just wooden in construction and still punched right through the knights armour.
Now earlier at creccy the english again faced a large body of french knights. this time the english had 12,000 men mostly archers while the french had four thousand crossbow men and 12,000 knights. The crossbows were too slow to reload and obviously lost the firefight and again plate armoured knights were driven into the mud by the english foot archers.
If those odds were different imagine mongol, turkic, or even crusade era arabs with composite bows that have a longer range and greater power against those knights! The result would be even worse becuase being nomads they would probably give chase after the battle and completely wipe out whole armies no problem!
The reason why comosite bows were rarely seen in such huge numbers and in mass volleyfire was becuase they were harder to make than self bows and require a high skill to produce them as well as being very expensive. That said whenever they were used en-masse they were very effective often the decideng factor in many battles.
Hope al this just helps clear up those indescrepensies. It is very much possible that westrern plate armour was developed in response to mongol attacks on eastern europe. however it seems more likely it was produced as a response to the wooden self bows of the europeans. However since most wooden bows can punch through classical plate and even kill elephants as the big game hunter bob swinehart showed by killing all the big five in africa. it is obvious that composite bows could easily do the same or better. hence why the recurve composite bow ist still used by many elite special forces today! The US has invested millions in modern bows based on the classical composites of the orientals. Also of note are the chinese repeating crossbow and italian arbelast (rifle crossbow) still used by chinese special forces as well as the italians and americans becuase it is cheaper and often more effective than silenced pistols.
Anyhow composite recurve bows are still used in the olympics. And while many types of plate armour are used u may be surprised to see many modern tanks fitted with large scale chainmail becuase it forces rockets to rebound on the weaker tank turret and body joints!