Abecause if you damage your head, you are damaging your brain, which is the CONTROL CENTER for EVERYTHING YOU DO in your body. If your spine gets damaged, it is a HUGE part of the nervous system, so if you damage your spine, your legs could also get paralyzed and your chest which would lead up to the arms and so on. That is why these two are MUCH more serious due to the further harm it does to your body.
