"There were 2 main reasons:
The Second Industrial Revolution created the need for a workforce in new factories,so many people moved from the countryside to the cities to find jobs in these factories.The effects of the depression of 1873-1896,which spread to America,further encouraged people to go to the cities to find work.
There was a long economic depression in Europe from 1873-1896,which encouraged many Europeans to emigrate to America.Most of these arrived at major port cities,and found jobs in factories so stayed there.