There is a constant battle between fletchers and armour smiths to one up each other. The armour smiths craft armour from stronger, sturdier and harder materials, meaning arrows must be made of equally, or greater, sturdiness of materials to pierce through them.
The adamant arrow is a shining example of this eternal conflict. The metal is durable, powerful and deadly in the right hands, and pierces mithril, steel and lesser metals with almost mocking ease. Only adamant and greater quality armour stands a chance against them.