Better life. Farming (say in the 1840's- 1850's when factories are becoming a big thing in the North, especially New England) was very hard. Plowing and scything is hard manual labor, all farm labor is constant and many farms in New England were not competitive with farms in Ohio, 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. There's also just so much land available for farming, and much of the wealth stays with the land- so, the third or fourth sons may not have enough money to buy a farm, and there may be no farm to buy, either. This did not prevent people from heading west to get cheaper land to farm ( Hamlin Garland's Main Traveled Roads is a good- and not too romantic- memoir of his father moving his family to do just that). But plenty still could not. 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.