Explain why the repolarization phase of the myocardial action potential correlates with beginning of diastole?

Actually, there's a little delay. Ventricular depolarization precedes ventricular contraction and ventricular repolarization precedes diastole. But the delay is in milliseconds. So, although diastole doesn't begin with the start of repolarization, by the time repolarization is complete, the cardiac muscle has started relaxing. The T wave typically appears during phase 3 of the cardiac action potential and isometric relaxation starts during phase 4 of the Wiggers diagram.