with the amount of cooldowns and natural damage reduction tanks just plain don't need the damage reduction from tenacity for like... anything
It's actually a rather strong stat but if you don't need the damage reduction for anything in particular why not take twice as much damage another stat might give

even if they make direct hit a dps exclusive stat and make tenacity be like 70% of what direct hit would have done in direct hit chance on top of the damage reduction
All that will mean to tanks is the double scaling of crit will bypass it 30% sooner than it would if you could use standard direct hit melds

Honestly as things stand the only way tenacity would be worthwhile is if the damage reduction got high enough that healers could get X% more glares in since healer damage is higher than tank damage to the point that tanks self healing so healers can dps more is a net gain already but players are greedy as hell and the concept of "if i stack enough tenacity the white mage can do 20% more dps while also increasing my own dps a little" would just be completely ignored