Now, on paper that's good; once you're past a certain skill level as a tank, the only things that'll really kill you are damage spikes; sudden big hits, your healers having to deal with something unpleasant at their end, the situations where Something Goes Wrong. On paper, extra survival cooldowns are great. Except...
Armour's only any use against physical damage; usually there's a heavy magic damage component to spiky tank damage (although that said, SoO has quite a few physical-heavy bosses). Strength's only a parry boost, and many major damaging abilities simply aren't parryable. Stamina - instead - basically acts to give your healers a moment or two extra to handle bad situations (or indeed yourself, if it comes about due to you being slow to do something that can be remedied),
The gem sockets, on the other hand, are powerful because they can be anything. Given the increase in focus on active mitigation, from a paladin perspective the single most powerful survival ability we have is reliable resource generation - we need to hit reliably and rapidly to generate holy power. As such, what benefits us the most is hit, expertise and haste; it's actually worth us going for the hard cap of expertise, which isn't true for many classes.