It is a function of both geography and history, both based on water. The line you are seeing in the east is the rain shadow of the Rocky mountains.

Moving east from California, you have the great basin desert, followed by the Rocky Mountains, and then the relatively dry great plains.

Population follows precipitation.

https://gisgeography.com/us-precipitation-map/

https://www.thoughtco.com/geography-shapes-us-regional-weath...