What is a likely reason an author chooses a particular theme?

So the theme of a book is the message it conveys. I would say the author chooses a theme to get a certain point across, maybe it's the authors way of giving their perspective on something.

the african society basically suffered the consequences of colonization and apartheid directly attacking the black people, first, during the colonialism and its main idea which which "white people" had better rights, affecting   them principally in the right to establish black people in their territories, access to quality education and health services and took away any civil right even to the black people to feel they have the same opportunities as the settlers.

a term is used to describe a sudden revelation in a short story is "showing forth".