Why are veins not blocked by plaques?
Atherosclerosis can develop in high pressure systems, such as arteries, and not in low pressure systems, such as veins. It is high blood pressure that can damage the lining of the blood vessel and initiate the process of atherosclerosis.

BUT, atherosclerosis can develop in veins that have been used as grafts instead of arteries, for example, in a coronary artery bypass surgery.

Springer: Atherosclerosis:


  In this regard, we may note that atherosclerotic lesions do not
  develop in the veins in their normal environment of low pressure and
  high flow, but that the lesions do develop when the veins are used as
  arterial bypass grafts where they are subjected to high pressure.
  Similarly, atherosclerotic lesions develop in the pulmonary arteries
  only in pulmonary hypertension.