I'm proud to be a third-generation native of Denver, but I don't know for sure why my ancestors first moved there. With one exception: my maternal great grandparents moved from New Jersey in 1891 seeking a dry climate after my great grandfather doctor caught pneumonia treating patients during the blizzard of 1888.
My mother's father was born in Omaha in 1896 while his Swedish immigrant parents were on their way to Denver, where there already was a Swedish community. Why they left Sweden and why they picked Denver, I don't know.
My father's father moved from a farm outside Detroit around 1888 and his mother moved from Michigan City, Indiana around 1902. One of her sisters was already living and working in Denver. They settled and married and prospered.
This weekend, I was reading an article that offered a possible explanation for their migration -- the lure of economic opportunity. I hadn't realize how rich the west was toward the end of the 19th century, but it turns out that Colorado was the third richest state in terms of per capita income in 1880 and 1890 and fifth highest in 1900. Colorado's population more than doubled between 1880 and 1890 and grew another 30% in the following decade. Much of this wealth came from gold and silver mining, of course, but the numbers suggest a booming economy that was widely shared.
Obviously much migration to and within the United States was driven by economic factors -- leaving declining areas and moving to growing ones. That's the best explanation I have so far for my own ancestors.