> This is not the case. Sounds like something that comes from a pop account of crecy or agincourt? Gunpowder is closer to the mark, but funnily enough armor kept getting heavier longer than many realize. The full plate harnesses that people generally picture when they think of a “knight in shining armor” are really Renaissance/early modern, well after the Middle Ages.

Really a combination of weapons (first crossbows - not because they were super effective against knight armor, but because it was cheaper to train crossbowmen, then gunpowder for the same reasons. Knights were reasonably effective well after primitive gunpowder weapons.) and gradual decline of feudalism and thus knights as a class, as Monarchs started transitioning to standing/professional armies.

Also, as manufacturing improved, it became much cheaper to equip soldiers with standardized equipment rather than rely on tailored to the individual hand-made full-plate armor.

As always, the truth is somewhere between economics and cultural/tech shift.