As mentioned before, it comes down to weapon damage and STR. Both increase the damage you do, and thus are both strongly affected by the emnity multipliers on Skull Sunder, Butcher's Block, Savage Blade and Rage of Halone (among some other).
The best data that has been parsed suggests a 3x multiplier on SB and SS, and a 5x multiplier on BB and RoH. This means an SS/SB for 100 damage gives you 300 emnity, and for BB/RoH gives you 500.
So if you take a fresh 50 WAR in all ilvl 50 stuff, he's hitting BB for the high 200's at most. That's between 1000-1200 emnity. At ilvl 86, my WAR is hitting BB for over 400 without buffs. That's over 2000 emnity, with crits up to over 900 with SE + Berserk or Unchained active, and topping 1000 sometimes when I've got SE + both up (aka ~5kish emnity). And I'm not even done melding my rings! In fact, at this gear disparity I could use an un-comboed BB (100 potency vs 280) and have more emnity per gcd than the ilvl 50 WAR, even if the WAR combo's everything correctly.
Then you add in Defiance, which is estimated at about a 2x emnity buff based on testing since the patch that buffed WAR (I know it says 25% more on the tooltip, but the testing on the tank forums states otherwise) and I can throw out 10k emnity per comboed BB vs the 2kish emnity for the ilvl 50 WAR.
In short, weapon damage makes a huge difference and STR isn't far behind. It used to be a good WAR or PLD could hold aggro with just the regular relic, but having a Zenith helped. Now, with higher gear levels and stronger weaponry, Zenith (or equivalent ilvl) has become the new baseline. A fresh 50 tank partied with geared DPS would either need the DPS to slow down significantly, or else would be well-served by acquiring a strong tank weapon before hand (Garuda or Titan weapon for instance, preferably Titan since it's ilvl 80).