Now, first of all, medieval armor wasn't simply a sheet of metal they stuck on your torso before marching you off. It was generally worn with padding beneath, for both comfort and safety. There's no point to "skintight" armor, and boob holders are even more unnecessary when you're swaddled up like the Michelin Man under there. Secondly, the whole point of armor is to deflect incoming blows -- as in, bounce them away from your tender bits. But if your breastplate has two big round mountains on either side, the enemy's sword is going to slide down them, right into the center of your chest.
As Tor's Emily Asher-Perrin explains, a strong enough blow to the chest can kill you, armor or not. Even falling down in a piece of unpadded boob armor would cause those metal cups to dig into your chest, resulting in some serious damage. If our boobless readers are having trouble picturing the problem, imagine a jockstrap that individually wraps the penis and each testicle separately, instead of putting the whole unit behind one single protective sheet of hard plastic. Then imagine somebody kicking it.