Its a fictional novel. And I believe its called The Stranger because.. Meursault is alienated and divided from the rest of society. Mostly because of his atheism and his detachment from reality.(:
This story is called The Stanger because Mersault is a stranger to many things and many people. He is unemotional toward the death of his mother, does not seem particularly upset during the funeral, and does not seem to embrace any of his cultural heritage. This is fiction, but it has a historical context.
This story is called the stranger because everything is foreign to Meursault based on how society perceives him.