Why was ziggurat not made of steel or wood?

Ziggurats were primarily constructed from mud bricks and fired clay due to the availability of materials in the regions where they were built, such as Mesopotamia. Stone was also used, but steel and wood were not practical; steel was not commonly available for construction in ancient times, and wood was scarce in the arid environment. Additionally, mud bricks provided durability and insulation, making them well-suited for the climate and the architectural needs of these monumental structures.