Determination offers more damage than Tenacity.

That's about it for why Tenacity is not favoured.

Tenacity will provide damage reduction which Determination does not, but it is often not necessary to use since it will rarely save a Healer having to spend an extra GCD on a heal to top you off.

In addition, even if it DID save the Healer having to spend a GCD on a heal, it only means that they'd spend the GCD on a damage spell instead. Which would be worth less damage than if you simply had a better stat instead of Tenacity boosting your own damage instead.

Even with all this, there's still only one Tank that actually wants Determination, which is Warrior. Other Tanks would prefer having Crit and Direct Hit for higher damage, while Warrior gets less damage from Direct Hit due to their job mechanics providing guaranteed Direct Critical Hits and so gets more damage from Determination than Direct Hit.