Most of daniel defoe's novels include major characters who do not belong to the upper class and suffer a life of hardship. what may have the writer to realistically portray such settings for his novel?

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.