Why don't people like to pick/use Tenacity over Determination?

[Gr1mwolf]

The big difference is in how damage vs defense is valued, due to enemies having insane health pools relative to players.
Having 5% more defense won’t allow you to survive a single extra hit without dying, which makes it functionally useless (I’m pretty sure this is also true of all the 10% DR cooldowns, which is strange). Say your tank would normally die from eating a mechanic followed by a tank buster; having that extra defense won’t stop that from killing you. If one mechanic takes like 70% of your health, and Tenacity reduces that to 67%, you’ll still die in the same number of hits.
Having a couple extra % of damage though actually will shorten the length of a fight, even if it is only by an insignificant sum.
It also won’t reduce damage taken enough to reduce the number of heals needed to get you back to full health.

The only way to make Tenacity relevant would be to either increase the damage reduction to a massive number, like 3x what it is now. Or make tanks have so much defense in general that keeping them alive is a non-issue, and extra defense just means more damage and less healing from the healers over the course of a fight.