User blog comment:AYET/Shanks missing arm/@comment-26195997-20120221105628/@comment-4597065-20120221112538

Actually, the reasons (to me) on why Ace died is:

1. Sakazuki's haki prevented Ace from using his Devil Fruit.

2. Magma just burnt the fire.

3. You have to know that Ace took the attack to the heart.... so when it was attacked, Ace became drastically weak and was no longer able to use his Devil Fruit due to lack of energy. So Ace could not heal in time and died.