I'd hazard a hypothesis and say it's because of the noses extra pressure sensitivity, when your nose is hit (the outward center of your sinuses, which connects either directly or indirectly every sensory organ in your head) the rapid pressure change is sensed along with the standard impact and pain. whats happening is two senses are being activated at once, versus being punched in the side of your face really only activates one type of sense, being "standard" pain, think of it like when you're eating and you can smell the food so the food is far more delicious. When you cannot smell the food you are eating, the flavor is much more mild and the experience is less rich because only one sense is being activated. when both are activated your brain combines the sensory information into one experience and makes it far more enjoyable, same with the nose being hit, two senses versus one being activated, so the experience is far stranger and almost has a third "dimension" to the pain.