Adamantine Armor says “any critical hit against you becomes a normal hit.”, but why does it effect spell crits?

@mikemearls @JeremyECrawford Adamantine Armor says “any critical hit against you becomes a normal hit.”, but why does it effect spell crits? Book says it’s reinforced armor from the hardest substance on earth. Why would a super hard ore prevent, dmg from a psychic spell crit?

— Kantrism (@Kantrism) November 8, 2017

Adamantine armor is a magic item. It’s not a chunk of ore. #DnD https://t.co/qUdnQrmpi7

— Jeremy Crawford (@JeremyECrawford) November 8, 2017