The blood supply... To surface area in these areas are low and exposure is high therefore they lose heat more readily. The ears are usually include in this group.
They're far. The most extreme portions of your body are called "extremities" for a reason. Since the skin on these parts are farthest from your heart, they get more easily compromised when the body is shunting skin circulation to preserve body core temperature. The thinner you are, the more this will occur. Also, some medications and medical conditions (raynaud's phenomenon) can cause this as well.