Qunari use vitaars instead of helms. Since vitaars increase damage rather than armor rating and a KE's main advantage is converting damage into barrier (which doesn't benefit from armor rating), Qunari KEs are far more effective than those of other races.
While this is technically correct, it's only part of the story. Part of what makes the Qunari a great Knight Enchanter is indeed the specialization's focus on using offense to power defense. The other part, though, is the fact that Qunari just make amazing mages in general.
The key is the Vitaar. See, when you look at a spell or ability, it says that it do s a certain percentage of Weapon Damage. The thing is, the Weapon Damage stat it uses isn't the one printed on your weapon; it's the one printed on your character sheet. Now, for the other three races, these are pretty much the same thing. Qunari, however, get to wear Vitaar, which adds it's damage to the base damage of the weapon they have equipped. This means that the Qunari has a higher Weapon Damage stat than an equivalently-specced Human or Elf Mage. I sat down and did the math on this. First, I used the starting staff and the first Vitaar you get in the prologue, then I checked with the best Vitaar and a Tier 3 crafted Staff using Tier 3 materials in the Primary slot. In both cases, the Qunari's damage was nearly 30% higher than a non-Qunari's! Of course, this number doesn't account for Attack, Crit Chance, Crit Damage, or the stats that affect those three stats; it's just pure Weapon Damage. Still, 30% more damage at pretty much all points of the game is pretty powerful.
Now, before you start running around declaring "Qunari Master Race!", there's something to be aware of. The only reason that the numbers are so big for the Mage is because Staves have such a low damage stat. The best crafted stave in the game in terms of just the Damage stat is a Tier 3 crafted stave using Dragon materials in the primary slot; this results in a damage stat of just over 70 (I want to say the actual number is 71 or 72). Warriors and Rouges beat this damage number long before end-game, so the damage added by a Vitaar ends up being a smaller percentage of their total damage. I haven't done the math on that, as I don't know the damage values of crafted materials for Rogue or Warrior weapons, but I have seen the overall damage increase for them estimated to be around 10%.