Why was the longbow so lethal?

Answer 1

Because it required little skill to fire, even though it did take a fair amount of training. A lord could more easily muster an army of peasants with longbows than train them to fight with swords and shields. This resulted in large ranks of men with longbows. So numbers. Numbers were why they were so lethal. That and the fact that if you can't defend against a hail of arrows and a cavalry charge at the same time.

Answer 2

The English longbow was a very powerful weapon, in terms of the kinetic energy it imparted to the arrow. Very few archers today use bows as powerful as the English longbow was, because using it took building up muscles over a long time. In fact, modern physicians can tell who the longbowmen were from their skeletons, because of the changes the bones underwent from the stress of using the bow.

The arrows shot from these longbows would pass right through the best armor available. The battles of Crecy, Poitiers, and Agincourt were decided by them, as English archers mowed down massed knights in armor, who were attacking them.