Improved life. In the 1840s and 1850s, when industries were starting to gain popularity in the North, particularly in New England, farming was quite difficult. Many farms in New England were not competitive with farms in Ohio and Indiana, where the soil was better, and once the Erie Canal came into use c.1830 and permitted easy transport through the Great Lakes, they found themselves in competition with them. Plowing and scything are hard manual labor, and all farm labor is constant. A farm may not be accessible for purchase by the third or fourth sons since there is only a certain amount of land that can be used for farming, and a large portion of wealth is retained with the property. This did not stop families from traveling to the west in search of less expensive property to farm (Hamlin Garland's Main Traveled Roads is a wonderful, unromantic memoir of his father moving his family to do precisely that). But many still were unable. The Lowell spinning mills found a very rich source of cheap labor in the hundreds of girls who wanted to leave the family farm- even if they wanted a farm of their own, their best hope would have been to marry a farmer.
But given factory conditions and wages, it is still a little surprising so many made the move. Southern apologists for slavery tried to contrast the harsh New England factory conditions with their (supposedly) well-treated slaves. Horatio Alger, in his many optimistic novels of bright young men always getting ahead, never had one getting ahead by working in factory. Probably if it had not been for immigration, industrialization would have been much much slower.
