Answer:
First, before I answer, note the definition of the word memoir. A memoir is a historical account or biography written from personal knowledge or special sources. In my opinion, Elie Wiesel chose to write his book "Night" as a memoir instead of a fiction book because this particular story is about Elie Wiesel's personal memories and experiences during the the Holocaust in the late 1930s to the early 1940s. Night is about Wiesel's personal story and what he had to go through in order to survive and get liberated by the Allied Powers (mostly by the United States and Soviet Union). So, in my opinion, the novel would have more impact on the readers as a memoir because Wiesel could incorporate his personal thoughts, feelings, and memories into his book, compared to a fiction that just states facts. Thank you! :)