Prior to the Industrial Revolution, most work and manufacturing was performed at home in a small scale. People usually produced most of the things they needed with their own hands. Whenever trade and commerce took place, it was on a small scale and at a local level.
This changed when factories became more common. The use of waterpower and steam engine made work faster, and the introduction of interchangeable parts allowed for a much faster production of goods. This made it profitable to manufacture products in an extremely large scale. It led to mass production.
Such changes completely transformed economic structures. This reduced the independence and relative value of craftsmen. Goods could now be produced by people who were not particularly skilled. Moreover, working hours, prices, production, and all other organizational details no longer depended on the craftsman, but on the factory owner. Wealth and population concentrated in cities, because this was where the factories were located. With the growth of cities, a new category of urban poor developed, with many people unable to find adequate housing or work.