The coronary arteries are under addition stress due the extra motion of the cardiac muscle which moves and stretches the coronary
arteries every heart beat, especially as the
arteries branch off from the aorta which is relatively stationary, while lower down over the surface of the heart, the vessels move vigorously with each heart beat. Bottom line hard plaque is essentially the net result of the healing process for these little cracks in the arterial wall resulting from mechanical stress.