Why can't porcupines shoot their quills

I want to know WHY they can't shoot them, not if they do or don't only.

They cannot because of physics. Force = Mass x Acceleration. The quill has mass, and it needs a force to accelerate (moving from being at rest to full speed = acceleration) out of the porcupine's skin. The porcupine's skin cannot generate a large enough force to propel the quill out of the skin. The quills are barbed, and if they are stuck in the skin of a predator then the predator pulling back away from the porcupine supplies the force needed to separate the quill from the porcupine.

If the porcupine were a bird, and its quills are attached to the wing, then the flapping of the wing may provide sufficient force to shoot the quill.

They are modified hairs that grow in the skin, just like your hairs. There is no mechanism for "shooting" quills, but they are barbed and will pull loose from the porcupine when the other end is stuck in some other animal.