check any forum dedicated to tanking that isn't this one. blue gartr is one decent place to start, official forums tend to be one of the worst places to look for solid info (we have many more threads in the tank forums here talking about how wearing dps gear is better for tanking, and how marauder/warrior dps is some of the best in the game)
it really boils down to eHP, which is a concept foreign to the majority of posters here... i wasn't really aware of what it meant/how it worked either until i started reading the tankspot forums/guides years ago.. (a better place to start if you're curious about the mathematics behind it)
the super dumbed down reasoning is this: you gear yourself to have the highest possible TTK (time to kill), there are thresholds along the way for both mitigation and raw hp that make one or the other better than the other, but what we're seeing in ffxiv is that base mitigation is so high compared to our HP values is that it's almost never worth it to sacrifice vit for HP - i.e. sacrificing the 25 vit from ilvl90 of fending jewelry for the +45str and +45dex you'd get from fully melded ilvl70 jewelry isn't worth it, your eHP goes down by that trade-off
it's entirely possible that it'll be worthwhile to use str/dex jewelry after HP values hit that critical point though... but that's only assuming that the ilvl100+ tank gear doesn't come with some built in +parry/block rate/strength on it
edit: i want to say that the HP value for paladins to start looking for mitigation stats rather than raw hp is around 8k, but it's been months since i looked at the math for this stuff
edit edit: also, this is only assuming MTing, and only assuming you want to gear yourself as the most survivable tank possible - in this case of offtanking (cough turn 5) most of them (paladins and warriors) swap out fending jewelry for the of maiming/striking variety, the very dedicated even reallocate their vit to strength, but this has nothing to do with eHP and everything to do with the fact that their role in the fight is a dps the majority of the time