Because of the rain-shadow effect, where mountains wring moisture out of the air on one slope and leave the other dry
As wind pushes air up a mountain, the air cools and drops its moisture as rain on the windward slope. By the time it crosses the peak and sinks down the other side, it is dry and warming, so the leeward slope sits in a **rain shadow**. It is why one face of a range can be green forest while the opposite face is desert.
