More to the fact it's speed.

Due to how Enhance are pretty much 80% spell damage, and the axe is slow.

There's more elaborate write-ups, but basically (if I remember right,) Enh actually does MORE damage with this weapon hard-casting Lava Bursts between cooldowns then actually hitting with it.

And by MORE damage, I mean more then the guys who run 2.6/2.6 speed agi weapons, which there's a surprising lack of.

According to one of the dps aim sites, the top dps in full heroic t12 was an enhance shaman with a caster main-hand weapon. This was a similar scenario pre-cata I believe (can't fully remember haven't had my shaman that long) and was something the devs were specifically tryingto get us away from and use two agi weapons as intended. Unfortunately with the way our damaging abities work the way our hardest hitting abilities work, more spellpower can be a significant boost to our damage, moreso than intended use agi weapons. I don't think this is standard for very level of gear for enhance shamans (they always suggest using simcraft to find out) but I could be wrong. I play enhance/resto so I suppose it's something I could test myself...

AoE.

Flame shock and Fire Nova use Spellpower. This weapon is superior to a normal weapon when doing a heavy AoE phase because not only does it make those 2 incredible, but the Maelstrom Weapon powered Chain Lightnings will also do more damage. The result is that any lost DPS due to the weapon's speed and damage is made up for in the spell damage.