This is my simple history as some who never lived there but has a degree in history.

In the 1800s, Oregon, Washington, and Idaho were part of the Oregon territory. In the middle of that century, white settlers began to move in and they hated slavery and didn’t want it but they hated free black people even more than slaves. They eventually passed a few laws that banned “negroes” and “mullatos”. The area also had a large influx of religious fundamentalist and separatists in its early history.Those laws were eventually repealed but created the majority white population of that area.

The early/mid 20th century saw other issues like the KKK but those were minor compared to what comes later. Cut to the 1970s and the Aryan Nations and other white nationalist groups began to move there. Those groups grew and encouraged more like minded racists to move there in hopes of setting up a whites only homeland.