Here's why:

* Prevailing Winds: The prevailing winds in the Pacific Northwest are from the west. These winds carry moist air from the Pacific Ocean.

* Orographic Lift: As this moist air encounters the Cascade Mountains, it is forced upward (orographic lift). As the air rises, it cools, causing water vapor to condense and form clouds. This leads to precipitation.

* Rain Shadow Effect: The western slopes receive the brunt of the precipitation. By the time the air reaches the eastern slopes, it has already released much of its moisture, resulting in a drier "rain shadow" effect on the eastern side of the Cascades.