Explain why when a person’s cells freeze, the damage is often irreversible

Because the cell will expand and burst

When a cell is frozen, that means the cell temperature is going to be lowered extremely. Ice has more volume than water which is why it floating inside water. That's why if you put water in a plastic container then freeze it, the container will get pressured and swollen. If the container is not elastic enough(metal, glass), it will burst. 
Same like that, the cell that got frozen can be burst too.