This is beacause the left ventricular myocarium is the thickest. Hence the myocardial oxygen demand is maxiumum for left ventricle. Right ventricle and atria get some perfusion directly from the cavity by sinusoids and reverse perfusion through Thebesian veins. Left ventricle gets perfusion mostly in diastole while other chambers get perfused in diastole and systole. This is because the high intracavitary pressure in systole squeezes the intramyocardial blood vessels of the left ventricle in systole. Left ventricular epicardial region gets perfusion in both systole and diastole while the subendocardial region gets solely during diastole.
