Because artifacts are not always meant to be stronger than crafted weapons. Auriels Bow is not meant to be the strongest weapon in the game, it's meant to be a weapon able to blot out the sun or do extra damage against the undead.
Auriel's Bow is a light bow based on the Elven bow, I think, and so does less damage. The light bows draw and shoot faster but do less damage per hit. Not sure if there are any DPS calculations out there. Does it outdamage a Glass (or Dragonscale, if they exist) bow?