George said:
Doesn't exactly explain why but it really doesn't matter.
The primary reason is that Norwegian emigration to America didn't really kick into full speed until around 1850, and that most Norwegians who emigrated in that first wave were cottars. A cottar is basically a farmer without a farm. They often moved farms every few years, and worked for the owner of the farm. They were given a little bit of land to produce for themselves while they lived and worked there. The problem inherent in the system was that there was next to nothing passed from generation to generation--certainly no land.
So, we have a population of people who know how to farm, but don't have a farm. The first major wave of Norwegian immigrants settled in what was (at the time) about as far west as regular farmers settled---pretty much the Wisconsin/Southeast Minnesota regions. After the Homestead act passed in 1862 (I believe), word of free farmland made it back to Norway through people who had already immigrated. Also, the travel industry heavily promoted the idea of travelling to the USA for free farmland---railroads and steamship companies advertised heavily. The last 25 years of the 19th century saw Minnesota and the Dakotas become heavily settled by Norwegian farmers.
By the early 20th century, much of the good farmland had already been taken. Even in 1905 when my great-grandpa Oksendal moved to the USA, he had slim pickings for farmland in the area where he settled. In the ensuing years, children of farmers had to find some sort of career. As always, there was a finite amount of land, so some had to find an occupation other than farming. Many moved to Washington, which offered more jobs than the Dakotas and Minnesota. During the Depression, many failed farmers also moved further west in search of more stable employment.
Washington had a Norwegian presence somewhat concurrently with settlement in the Upper Midwest, largely due to ports and shipyards (Norwegians are also traditionally pretty good sailors). It also doesn't hurt that the climate of western Washington and coastal western Norway are very similar. Many who had initially settled in, or were born in the Dakotas and Minnesota found the Pacific Northwest more inviting.
From points in the Upper Midwest and the Pacific Northwest, Norwegian-American settlement dispersed fairly evenly across the western United States. In the case of the Dakotas, there has been a consistent outflow of population since the 1930's. These people tend to settle in western states, which explains why the western USA has a higher concentration of Norwegians than the east.
Of course, this is very much a macro-view. There were Norwegian settlements in New York, Pennsylvania, Texas, and of course in every other state in the union. The Upper Midwest, and the Pacific Northwest, however, were the main hubs of Norwegian-American Settlement and the bulk of the 4 million odd Norwegian Americans passed through these areas.
Most of this migration happened in the course of about 75 years, from around 1850-1925. WWI put a damper on immigration from Norway, as did the Great Depression in the 1930's. There was a very small burst of immigration after the end of WWII, but in the 60 years since the end of WWII, economic conditions in Norway have improved to such a degree that immigration to the USA is barely a trickle.
I hope this answers your question.
