Why were the streets de-peopled?

The short story "After Twenty Years" by O. Henry tells the story of two men that used to be friends who meet after twenty years. Jimmy Wells is a policeman out on his beat on a cold night, and Bob, his former friend, is now a wealthy criminal. The two men meet on the deserted street, but Bob does not recognize Jimmy. After Jimmy leaves, he sends another policeman to apprehend Bob, because he cannot bear to arrest his friend himself.
According to the Collins English Dictionary, the verb "depeople" means "to depopulate" or "to reduce or remove the population of a place." When used as an adjective, as in this story, if a place is referred to as "depeopled," it means that it has been emptied or nearly emptied of the people that are usually there. In "After Twenty Years," the expression refers to the streets on which the policeman walks his beat. The reason that the streets are depeopled is made clear in the sentence the word appears in.

The time was barely 10 o'clock at night, but chilly gusts of wind with a taste of rain in them had well nigh depeopled the streets.

So we can see that the streets have been depeopled, or made nearly empty, because of the weather. Cold winds and approaching rain have caused most people to seek shelter indoors.
https://www.collinsdictionary.com/dictionary/english/depeople