[Galaxity]

Well I'll provide some thoughts and speculation based on my reading.

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.