If cast iron is such a hard metal why dont the military use cast iron to armor their tanks? Is it cuz cast iron is such a heavy metal that itll make the tank slow? Or is there not enough of cast iron on earth?

Weight is a really important factor in military applications. tanks need light durable and these days intelligent armour systems. most armoured personel carriers only have aluminium walls. Tanks these days have reactive armour and laser systems to neutralise incomming threats, rather than just relying on heavy armour.