You Have to Basically Hold a Knight or Squire in Chain Down While You and Your Buddies Stab Him Repeatedly for a Few Dozen Minutes to Hope to Kill the Knight Inside
There is butted chain, and there is riveted chain. Per the research I've done, nearly all chain actually recovered that is believed to have seen real warfare is riveted chain.
Here is a video of someone testing riveted chainmail against a variety of 12th and 13th century weapons, including towards the end weapons custom-built to defeat riveted chains. As you'll see in the video, nearly every weapon is ineffective. Even spears basically required multiple hard stabs to create the tiniest cut. This means, against an armored knight in combat, in order to kill someone wearing this kind of protection, you needed to hold them down and stab them repeatedly... like an awful lot.
Chain Mail is Prohibitively Expensive for the Poor Adventurer
According to some other articles, squires and knights alike (full time soldiers) wore the same level of protection. According to the Medieval Price List, a suit of chain cost between 1,200 pence (100 shillings / 5 pounds) to 1,920 pence (160 shillings / 8 pounds). This was equal to between 4 and 6 years wages for the common field hand!
Cheap weapons, at a few pence, were much easier to provide than good armor.
Veterancy Was Important
Per several stories, not the least being the Battle of Hastings, the way in which temporary troops were rallied is informative: a king would literally go into the local pubs and recruit. Or, the king would send messengers to their nobles who would go into local pubs and recruit.
Your average "infantryman", then, has maybe held a sword once in a county fair. Maybe he bought one. He may know how to throw a punch or use a knife or bow to kill game. He's is unarmored, barely armed, and as far as morale goes only around for as long as he stays motivated to hang around.
There were other types of armor around in the 12th century: like scale mail which was tougher than chain head-on, but if you were trained, could be defeated easily with upward strikes only. Or lamellar armor of tiny plates all stitched very close together, which was best attacked in-between plates.
A professional soldier likely knew these things.
An army of 20 thousand hastily recruited volunteers who'd been assembled so quickly that you couldn't feed them very likely did not know these things. Or, even if instructed, probably had not really internalized what they needed to do when a certain kind of technology was up against them on the battlefield.
Oh, By the Way, There was No Special Stroke for Chain
Until specialized weapons were invented with long narrow points to go inside the rings, there was no up-stroke, or between-the-joints stroke to defeat chain.
In the 12th century most European soldiers that were provided it wore long waist-length and arm covering sheets. The legs and hands were unprotected on the average soldier, although some spared no expense to cover hands, feet, and legs.
As the chain mail hit its sunset (because special weapons were being created to defeat it) plate began to roll out to defeat the new special weapons.