Possibly because the whole idea of a double axe is just silly?
Setting aside the fact that a weapon designed to maximize the force on the cutting edge through using a lever effect suddenly works (less than) half as well if you grip it by the middle......
I think that the double axe had sufficient benefits (higher damage die than most double weapons, usable as both two weapons and a two-handed weapon) that the defensive trait may have been viewed as unnecessary. In addition, very few higher-damage weapons are also Defensive in general, which may be a genre convention that WOTC wanted to promote (the heavy hitter is reckless, the finesse guy is careful).
Also, just looking at a double axe: what would make it more defensive than a regular axe? It's not like it has a smaller, more maneuverable parrying head, or a quick, jabbing point to push back an opponent. It's an axe, with an axe!
If it had been Defensive, too, I think it would have been way too attractive as a weapon choice (even to my own dwarf fighter, despite my aversion to weapons which I cannot imagine how you would wield....how do I describe my attacks if I can't figure out how anyone could possibly attack with this thing?).
