The materials their armor and weaponry was made of.

Back in medieval times all weapons were made out of iron or steel. Now around the 15th century was when tempered steel came into play which was stronger than the materials they were using before and this tempered steel meant they could reduce the blade width without taking away any strength from the blade making the weapons easier to wield as they were lighter. Around this time most arrowheads made were made out of iron and extremely sharp which rendered chain mail useless. On another point if you don't know what chain mail is it is a type of armor that was good for fending off slash attacks from the enemies and is made out of a bunch of steel chain links hooked together in the shape of a shirt. Chain mail was rendered useless against any arrows or stab attacks because the point of the arrowhead or sword would just slip through the links and keep going causing major damage to the person wearing the chain mail.