The answer is because when the heart pumps blood in the artery during systole it counteracts the resistance provided by the blood vessels. This requires high pressure that is provided by the force of ejection due to the contraction of the ventricles. On the other hand, the pressure is lowest during diastole which is the ventricular filling of blood. There is low resistance in the blood vessels and the arterial wall pressure declines until the heart starts to contracts again.
