Why can organs of a brain dead patient be used for transplantation and not those of a patient who has died of a cardiac arrest?

Solid organ donation (heart, lungs, liver, pancreas, kidneys) requires blood circulation to be maintained in these organs until retrieval. This is possible in brain-stem death where the functioning of these organs can be supported for some time.

However organ after cardiac death can also be harvested, provided the time gap is minimal.