[scythe]

That happens because there are many hills which tend to spawn rainclouds when the humidity hits a certain threshold. So while there is moisture in the air it mostly falls in the mountains. Other places with this pattern include e.g. Morocco or the Snake River Valley in Idaho.

https://en.wikipedia.org/wiki/Sky_island

(This is also why people keep building houses in fire danger zones: the forests have a nice climate, but they burn. The flat areas don't burn, but they're inhospitable.)