Why was there a massive amount of immigration during the period after the American Civil War until World War 1?

Immigration to the US has long been driven by political unrest and disadvantage in other countries, as well as the promise of a better life in America. In the earlier part of the nineteenth century, the US received a significant influx of Irish and German immigrants fleeing turmoil caused by the Irish Potato Famine and the wars in Germany leading up to its unification. However, the Civil War caused a cessation in immigration to the United States. This continued into the 1870s, as the postwar depression and difficulties with reconstruction meant that the US was not an attractive prospect for would-be immigrants.
From the 1880s to the First World War, however, the US once again experienced a significant wave of immigration. There were several reasons for this, but one of the primary ones was, again, political unrest in Europe. During the late nineteenth century, regimes in Central and Eastern Europe drove many Jews to seek refuge elsewhere. Anti-Jewish pogroms, or prejudicial attacks, reached a peak during this time, which resulted in a large number of Jews entering the United States.
The US also received an influx of immigrants from Italy. Italian Americans now represent a significant proportion of the US population, and the vast majority of this population arrived in the second half of the nineteenth century and the early twentieth century. The Italians primarily came from southern Italy, which was an agricultural area; there was not a particular political situation in Italy that provoked immigration, but an idea of America as an opportunity for a better life had been presented to them, particularly in the wake of Slav and Jewish immigration. The immigration of this period, comprising Slavs, Jews, and Italians, has been called "New Immigration."
Part of the reason the US presented such an attractive prospect at this time was that its industry was growing at a rapid pace. With the expansion of the railroads and the rebuilding of industry after the Civil War, even peasant farmers from southern Italy came to feel that the US was a place which would offer them more opportunities than they could hope to achieve in Italy, particularly in the rural south. A divide in Italy meant that it was difficult for southern farmers to aspire to anything outside of their region, and they lacked education; they hoped that in the USA, they would be able to receive a better education and better jobs during the industry boom.
Immigration slowed down again before the First World War, as the restrictions upon immigrants entering became more stringent. Then, with the outbreak of war, conflict once again put a stop to the global movement of people.
https://www.history.com/topics/immigration/u-s-immigration-before-1965

https://www.mtholyoke.edu/~molna22a/classweb/politics/Italianhistory.html

https://www.myjewishlearning.com/article/jewish-immigration-to-america-three-waves/