Why is? a?a broken neck instantly fatal or is that just a tv thing?

Medulla oblongata: A cervical or neck fracture at the highest level will crush or compress the medulla oblongata, where the center for respiratory drive is housed. This ceases to function in the moment of the injury, along with all functions lower than the level of the injury. This is incompatible with life.

Cervical nerves: Paralysis of the diaphragm- the muscle to breathe gets it's innervation from the upper spinal cord through cervical nerves 2, 3, and 4. So fracturing the spine and injuring the spinal cord high above these nerves causes loss of respiration (like christopher reeves) and death. The instant death occurs in craniocervical dislocation- the spinal cord becomes torn at the junction of the brain.

Shocked medulla: Disrupting the medulla, at the junction of brain and skull, by separating them ("internal decapitation") will relax the veins of the body, blood will not return to the heart, and the person is thus dead -- zero cardiac output -- before the body falls to the ground.