Because Citrate in stored blood products acts as an anticoagulant by binding ionized calcium, potentially causing hypocalcemia that leads to impaired clotting, arrhythmias, and reduced cardiac contractility.
Supplementation with calcium gluconate (often 1g per 3 units of RBCs) helps restore calcium levels, especially during massive transfusions or in patients with impaired citrate metabolism like liver dysfunction or hypothermia.