Final answer:
The main reason why a person dies when radiation exposure causes hematopoietic death is because the immune system is destroyed.
Step-by-step explanation:
The main reason why a person dies when radiation exposure causes hematopoietic death is because the immune system is destroyed. Radiation exposure damages the rapidly dividing cells in the body, including the white blood cells that are crucial for the immune system's function. As a result, the person becomes more susceptible to infections and illnesses, which can ultimately lead to their death.