Why do forests grow on the western and eastern slopes of the Cordilleras, in the intermontane areas are deserted?

The humidity or aridity of the region directly depends on the relief. Since the Cordillera rise higher than the clouds that carry moisture, they become an insurmountable obstacle in their path. For this reason, precipitation falls on the outer slopes, giving rise to a rich flora there. And in the intermontane space it remains dry, often deserted. This phenomenon can be seen in another example, for example, the Himalayas, the southern slopes of which are covered with moist forests, and the north with desert or semi-desert lands.