The force of gravity, the pressure of body weight, and the task of carrying blood from the bottom of the body up to the heart make legs the primary location for varicose and spider veins. Compared with other veins in the body, leg veins have the toughest job of carrying blood back to the heart. They endure the most pressure. This pressure can be stronger than the veins' one-way valves.
This answer is based on source information from The National Women's Health Information Center.