Why are the lower legs vulnerable to varicose veins?

Leg Veins. Varicose veins occur most commonly in the lower legs because when we are standing the pressure will increase in the veins of the legs if the valves in these veins are not functioning properly (reflux). The result is that the blood will back up and the veins will enlarge to cause varicose veins. This does not happen in the arms.
Lower leg veins. Usually varicose vein disease is genetically acquired. Weak vein valves progressively break as a result of venous insufficiency. Leg ultrasound often reveals venous reflux disease in veins called saphenous veins, which causes the development of the surface veins. After the vein valves break, abnormal high pressure develops in the leg veins. Gravity causes highest pressure in the lower legs.
Higher vein pressure. Veins enlarge in response to increased pressure. When veins do not work properly, gravity allows blood to back flow into the legs (reflux) when standing ,or even sitting with the legs dependent. This pressure results in dilation of abnormal veins---- varicose veins.