It's not really that the kuyak-ish medium armors are some kind of optimal god-teir armors, it's just that they fit in really well with the standard setup when it comes to price:
If you use a top-teir melee wep (Greatsword/Poleaxe/Top 1h+shield), a Kuyak-ish armor, and sensible boots, gloves and headgear you're going to end up with a total cost of about 30k, which is pretty much optimum since it allows you to make a steady amount of cash while using afromented gear (which is good gear).
Heavy armor is really good. I've tried it. It allows you to take a crapload of hits with the right build, but it's oddly weak to ranged thanks to extra_penetration.
But the main gripe with it is that it doesn't really fit cost-wise. If you're going to use heavy armor you're going to have to cut down on something else to make money, and what? Weapon? Noone wants to use a crap weapon, especially not in heavy armor since the style favours long/heavy hitting weps (the expensive ones). Boots/headgear? Will make for an obvious target.