Final answer:
Settlers on the Great Plains often lived in sod houses due to lack of traditional building materials.
Step-by-step explanation:
The major reason why settlers on the Great Plains often lived in sod houses was lack of traditional building materials. The Great Plains had very few trees, making it difficult to find lumber for building houses. However, there was an abundance of prairie grass, which could be used to construct sod houses. Sod, cut into large rectangles, was stacked to make the walls of the structure, providing an inexpensive, yet damp, house for western settlers.