Subject matter, I think. And location.
No one has mentioned James Jones, and I'm not saying "From Here to Eternity" should even be in the top 100, but I read that book in 1951 when I had been in the Army about one month, and I saw those characters all around me. The young regular Army guy who bunked across the aisle from me was one of them. Years later I was astounded to learn by accident that he had been executed for murder. He had killed three law officers during a domestic disturbance. If I ever write a book, I'll put him in it.