Why did the adichie choose to write "a private experience" in third person?

The story is narrated in third person in order to adopt an objective and neutral point of view about the situation, the idea of the author is to explain about the social problems presented by the different ideological and religious differences.

Explanation:

"A private experience" is a story of the compilation written by the author Chimamanda Ngozi Adichie in her book The Thing Around Your Neck. This story narrates a parallel story that has to do with racism and ethnocentrism in the United States. While outside in the streets, two men are fighting because a christian boy passes over the book of the coran with his car and the second guy  claims him for this act of infamy that runs over his Muslim religion. Due to this riot, a christian girl, Chika enters a store to feel safe, and inside the store there is a Muslim girl, Housa. While Chika waits inside the store, they have a conversation in which they realize they can help each other. Chika mentions she is a medicine student, while Housa explains her medical issues.

he or she may have a different tone of feelings.

you need to explain that more, your grammer just needs to be close to perfect.