The key reason for the Great Basin's aridity is the rain shadow effect.

The Great Basin’s dry climate is largely due to the rain shadow effect. As moist air from the Pacific Ocean moves toward the basin, it encounters the Sierra Nevada mountain range. The mountains force the air to rise, causing it to cool and release moisture in the form of precipitation on the western side. By the time the air reaches the Great Basin, it is dry, resulting in little rainfall and arid conditions. This effect, along with the region's geography, creates the desert-like environment typical of the Great Basin.