It’s about distance. The northeast is closer to Europe.
I can understand the areas that border the Pacific Ocean, but what was up with the high immigration to desert states like Nevada, Arizona, Utah, etc.? Who would want to immigrate from (presumably) the green British Isles to the desert? Western Oregon and Washington would be more their climate.
The high migration/immigration rates in Nevada, Arizona, and Utah in the late 1800s is relative. There was such a small amount of people that it was easy to have a large percentage from out of the US.
Who was immigrating there? Well one group was the Mormons. Apparently the Mormons were successful early on recruiting in Britain and those people migrated to the US and directly to Utah. Later that flowed over into Nevada and Arizona.
Also early on in these western territories, many people were probably going there to mine which may have attracted overseas people directly, not so concerned with green fertile land.
Final thought. The people immigrating from Britain and Ireland were looking for land. They probably had little idea what the southwest US was like except they had been told that there was lots of land available. When they got there it was totally different but there was land and water. Also many may have been going to California but stopped on the way for various reasons.
Edit: also those states are known for their population in deserts but they do have a lot of non deserts: forest,grassland, etc. And many early settlers stopped in those areas before geographic forces started the big desert settlements.
But even setting aside the massive European colonial machinery, the average stay-at-home 18th or 19th century European already had plenty of ideas about what people of non-European races were like, from the stories of colonial wars, tales of explorers' encounters with savagery, and general mythology and exoticism of what the rest of the world must be like. And those opinions were not, let's say, generally very enlightened.
You can still be a racist even if you've never met someone of another race. If anything it's much easier that way.
Yes, very small fraction of Europeans was involved in that. Overwhelming majority never left their country, and often their village.