Here's why:

* Brick: The majority of the Great Wall is built from brick. Bricks are made by firing clay, which is a sedimentary rock.

* Stone: While some sections of the wall use stone, this stone is also often quarried from sedimentary rock formations.

While igneous rock can be found in China, it's less common in the regions where the Great Wall was constructed, and it wouldn't have been as practical to use for building the wall.