Why do tanks need Determination stat still when they have Tenacity?

[Bourne_Endeavor]

This raises the next question; why should tanks sacrifice that 5-11% damage reduction that also increases DPS by 5-11%(Tenacity) for a flat 7-12% DPS increase(Determination)? Me personally; I would go for that stat that gives me more DPS and mitigation over the stat that is just raw damage.

Because mitigation only matters to a threshold. Anything above it is simply wasted stats unless your healers get an extra GCD out if it which will not happen. This is why Sentinal is objectively the worst of the big three tank CD. There is nothing that Vengeance or Shadow Wall won't cover. Therefore, making the extra 10% on a longer cooldown a worse trade. You essentially gain nothing but have to wait a full minute longer.

Tenacity would work better in WoW, where outgoing damage comes very quickly and consistently throughout the fight. In FFXIV, auto attacks aren't that threatening and busters are entirely predictable.

Simply put, Tenacity isn't bad, you'll just never have any real need for it.