How does Jimmy Doyle die as a fight injury?

In 1947, Doyle challenged Sugar Ray Robinson for the World Welterweight Title. Robinson had the advantage in every round except the sixth, when he was staggered twice and hurt. A single left hook from Robinson ended the fight. "That punch knocked Jimmy rigid..With heels resting against the canvas as if hinged, Doyle's body went down. It struck the floor with a thud, like a rigid mass falling. His head crashed against the padded canvas, and as the referee started the count. Doyle raised his head and rested on his elbows... The referee counted to ten. Doyle was out. Doyle was taken to St. Vincent's Charity Hospital immediatedly after the bout, and failed to regain consciousness and died a few hours later.