Will as James wrote above its the material used as well as the positioning. God forbid should Marc have a decent sized rear shunt, what is to say that the tank wont split with fuel being released inside the car.
If it was a bag tank it would simply adjust its shape and not split, if it was kept aluminium but moved to be inside the confines of the cage it is less likely to get damaged, or if the same location is to be kept, lower it so the tank is not in the cockpit, it would be a much safer setup.