I am not way too sure, but I will give a shot at the answer.
The loss of a heartbeat only determines a loss of functioning in the cardiovascular system, which is not responsible for all functions of the body. Brain death is when there is no activity regarding the nervous system, which is responsible for the functions within the body. Loss of cardiovascular functions do not determine brain death for this reason, as it is not the brain that has stopped, but the heart.
