Why was there a massive amount of immigration during the period following the American Civil War all the way until World War I?

This period in American immigration is called "New Immigration." There was a lot of immigration to the US in the early nineteenth century also, but the Civil War put a stop to this. A significant proportion of the Irish-Americans and German-Americans now living in the US are descendants of people who immigrated to the USA in the first half of the nineteenth century.
The immigrants who began to enter the US after the Civil War were generally from different parts of Europe and immigrating for different reasons. It was during this period, after the economy had begun to recover from the depression caused by the Civil War, that American industrialization really took off. It became commonly known in Europe that there were lots of opportunities in the US for tradesmen and also for those who were not skilled in trade to learn new trades, as there was a high demand for workmen. This drew a significant number of immigrants from farming areas of Italy to move to the US, as they felt America offered them opportunities they could not find in Italy.
Immigration from Europe in this period was also driven by persecution of Jews in Central and Eastern Europe, as well as in Russia. A huge wave of Jews immigrated to the US during the New Immigration period as a means of escaping hostile political situations in their home countries.