Trip to Hanoi - Fiction/Nonfiction

"Indeed, the problems was that Vietnam had become so much a fact of my consciousness as an American that I was having enormous difficulty getting it outside my head."

I was just looking up Susan Sontag (author of Trip to Hanoi) and an interview of hers caught my eye. Here's what she said:

Susan Sontag: Fiction writers have been made very nervous by a problem of credibility. Many don't feel comfortable about doing it straight, and try to give fiction the character of nonfiction. That a document of the writer's own character and experience seems to have more authority than an invented fiction is perhaps more widespread in this country than elsewhere and reflects the triumph of psychological ways of looking at everything. I have friends who tell me that the only books by writers of fiction that really interest them are their letters and diaries.

I think that's a really interesting rhetoric observation. I noticed that as a reader, I enjoyed reading Trip to Hanoi and was persuaded by it because subconsciously, I knew it was a first person account. It was something that actually happened. It made it seem more personal, and somehow more real. I remember when I read the Diary of Anne Frank, what hit me about it the most was the truth in it.

Why is it like this though? Why is it so important for us to think of something as true/nonfiction for it to have an impact on us? Does fiction not have the ability to touch our hearts or persuade us as well as nonfiction? Is rhetoric then dependent on truth more than imagination?