What caused the fall of bataan in World War 2?

Tactically, a Japanese attack which broke through the American-Filipino line. The breach of the line was serious and the defenders were not able to repair the rupture. This put the Japanese in a position to "roll up" the defender's line.

The location of this line was the last defensible place on the Peninsula. There was nowhere left to go, no next position to fall back on and resume defending. The defenders had been on half rations since arriving in the peninsula four months earlier, and soon after that they went on quarter rations. Starvation accumulating over weeks, the ravages of disease, men lost in battle, had thinned the defenders and stretched their line to the breaking point. Supplies of ammunition and medical stores were inadequate. The defenders had no hope of resupply or reinforcement, and did well to hold out for four months.