Everybody makes big deal of diversity in US. It is an accident of history. Unlike Europe, which has been settled for millennia, America was virtually empty when it was discovered by Columbus in 1492. The Indians were hunter-gatherers who had created only a few cities or towns in North America, mostly in the Southwest, although they had created grander ones in Central and South America. In North America there was relatively little resistance to the westward expansion of Europeans across the continent. There was never much threat from Indians against European-built cities after the first hundred years or so. As the Indians were driven westward, the war against them moved westward to protect the settlers as they moved in.
The westward expansion essentially created free land for those who were will to claim it and fight for it. This became the social and economic safety net for Europeans who could not make it on the more civilized east coast. If you couldn’t make it in Boston or Charleston, you could set out for Indiana or Alabama, and eventually Kansas, Texas, or California. Life was hard, but it was possible to get out of the oppressive slums in the east coast cities where immigrants first arrived. Today, if you are stuck in a slum, there is no wild West to go to. Three is no more free land, although people like Cliven Bundy claim that there should be. As a result, it is harder for people trapped in slums to get out.
Another mass migration that took place later was the movement of blacks from the deep South, where they had lived since slavery, to the industrial north, where low skilled jobs with good pay were available, particularly in the car industry in Detroit. These jobs became the security safety net for struggling poor people in the South.
When the Great Depression hit, however, the geographic safety net had largely disappeared. There was no golden region of the country to which people could flee for a better life. It was only then, under FDR, that the government moved in to provide its own safety net in the form of the CCC, WPA, TVA, Social Security and other government programs. These programs became necessary because by 1930, the formerly empty United States had filled up with people.
Prior to this there had been few restraints on immigration, because people saw it as positive to make use of empty land by farming, ranching, mining or manufacturing. During this open immigration period, most of the immigrants came from Europe, mainly from western and northern Europe. Thus it was not surprising when prejudice grew up against immigrants from Ireland and Italy by settlers of English and northern European extraction, for example. The descendants brought some of their old-country hostilities with them. Irish-English animosities were alive and well in Boston and Belfast well into the 1990s.
The idea that the United States has always been a land welcoming any immigrants from anywhere is largely fiction. Blacks arrived as slaves. The Chinese were discriminated against for years, as were southern Europeans. Even immigrants like the Germans and Poles, largely went west to more open places like Michigan and Minnesota, finding the already crowded east coast somewhat hostile to them.