Because neither the DP (Dragon Punch) or fireball are performed with the feet.
Edit: If the "fireball" in question is the Hadouken, of course. Otherwise, if the "fireball" is just being used as a blanket term for "projectile", then it's a different story.
"If the "fireball" in question is the Hadouken, of course. Otherwise, if the "fireball" is just being used as a blanket term for "projectile", then it's a different story." - @Hazard 2017