Here's why:
* Decoy's effect requires it to be face-up. The text reads: "When this card is targeted by a card effect, you can discard 1 card; if you do, negate the effect."
* Face-down monsters cannot be targeted by card effects. The effect of Decoy only activates when it's face-up and targeted by a card effect.
Therefore, if Decoy is face-down on the field and attacked, it will not activate its effect. It will simply be destroyed by the attack.