Migration of rural to urban and mass immigration.
People moved from rural areas to the cities because there were job opportunities in the cities. Farms were becoming more efficient, needing fewer workers. Workers were in demand in the cities to work in the new factories.
The factories were in high demand of workers and immigration aided in this demand. Many people in Europe were leaving due to harsh economic conditions and nationalist movements. These immigrants were in large numbers and flocked to the cities of America.