Why can’t you stack AC methods?

@JeremyECrawford Why can't you stack AC methods? Some foes end up with enormous hit bonus. Many things use VS a stat so it seems reasonable

— Dude Manguy (@SolarFlairPony) October 15, 2016

AC calculations don't stack (you have AC X + Y), but bonuses to AC do (like from a cloak of protection and a ring of protection). #DnD https://t.co/NZkW7cnfAb

— Jeremy Crawford (@JeremyECrawford) October 15, 2016