This is because he himself came from a middle upper class family and he had many acquaintances from the lower classes.
Defoe himself didn't come from a poor family - I wouldn't say they were rich, but they didn't have many hardships in life, at least not financially. Even when he grew up, he became a merchant so he had some money of his own. However, in order to write stories about such characters, Defoe had to spend time with them, with the lowest classes and get acquainted with their way of living.