It's simple really. It's because it's the ONLY weapon that monks can equip. Granted dwarven axes could use a bit more love, but your dwarven fighter also can equip other weapon types (if it's decent enough) without a penality.
In this case, even if you specced for dwarven axes, you could use any slashing one-handers as well, so: axes, bastard swords, handaxes, kopeshes, kukris, longswords, scimitars and sickle. That's 8 options that, while maybe not optimal, doesn't penalize you and, with good stats, could be better for you. If you count all those, you got way more than 3 options in update 14.
If quarterstaves and kamas didn't horribly cripple monk abilities so much, they could offer more potential options, but that's not gonna change anytime soon...