Hits to full armor aren’t about cutting through or breaking the armor, but trying to get as hard of a hit as possible. One central outward curve will deflect hits.
Two bulbous curves next to each other will deflect hits on the outside but focus hits on the inner curve into a shape that will ensure all of the power in the hit goes right to the chest. A great way to ensure the wind is knocked out of the wearer.
Plus, the biggest concern regarding the deflection of blow by plate armor was lances. A lance blow is often described as having the momentum of both the rider and the horse behind it (not quite true, but certainly more than just the knight). That is a lot of force for a body to absorb