Mass literacy only occurred at this time because of the urbanization that was brought about by the Industrial Revolution.
When people are largely farmers who are working without machinery on scattered farms, it is difficult to have mass literacy. Too many people do not have good access to schools or the time to attend school if they had access. With urbanization, this changes. People are more centralized in dense groups, making it easy to reach schools. With industrialization, more and more people come to have enough wealth to allow their children to go to school rather than working. These factors meant that the urbanized and industrialized society of the 19th century was the first to have the conditions in which mass literacy was possible.