Why wasn't the ziggurat made of wood or steel?

Ziggurats were primarily constructed using mud bricks and stone because these materials were more readily available in the regions where they were built, such as Mesopotamia. Wood was scarce in these areas and less durable against the elements, while steel was not yet widely used or available in ancient times. The choice of mud bricks offered better insulation and stability, crucial for the massive structures that needed to withstand the test of time.