How does hypertension cause an insufficient supply of blood to the heart?

It does not: Hypertension does not cause in a direct form insifficient blood supply to the heart. Hypertension is a risk factor for coronary artery disease, narrowing of the blood vessels that supply blood to the heart.

Several ways: Heart feeds itself with blood only during relaxation phase of heart cycle. High diastolic pressures impede this blood flow. Very high blood pressure acutely increases work and oxygen demand of heart and can potentially raise oxygen need above ability to deliver. Longstanding HTN causes heart muscle to enlarge creating a larger oxygen demand and a stiffer pump that is harder to fill.

Hypertension: Overgrowth of heart muscle relative to blood supply, damage to vessels.