The most accurate answer to the question is:
The clay bricks and mud the Sumerians used as building materials were not very durable.
This reflects the fact that the primary materials used by the Sumerians for art and architecture, such as clay, are more susceptible to erosion and decay over time compared to the stone used by the Egyptians. Consequently, fewer artifacts have survived from Sumer compared to other ancient cultures that utilized more durable materials.