Theoretically all veins are vulnerable, but it is in the veins of the lower extremities that it occurs most commonly. It is not entirely clear why it happens, but some patients with other underlying conditions, or some people who have been seated for a very long time in a car or airplane, get clots in the veins in their calves.
Blood does flow quickly in the upper extremities.
For example, contrast injected into the superior vena cava, a main vein in the chest, disappears in a flash as it is diluted by the on-rushing stream of blood. In the lower extremities, this is not the case. Dye in the veins of the calves can take forever to flow back up the leg and into the trunk and then into the heart. The veins are given some help. The calf and thigh are both very muscular.