The Gold Rush became the formative event in California history, but I think the Donner Party experience is like the dark side of the Gold Rush, the other side of the dream. And it looms large because the California Dream is so large. Other books still focus on the cannibalism and regard that as the deepest moment of the story's appeal. On a certain level, that is why the story of the Donner Party stayed alive. But in my view, that is not the source of the holding power.
Some people become savages.
Some people become heroes. In the end, it tells us something about ourselves. Even though you and I may not be pushed to that limit, it tells us something about human nature. The one who deals with that is Patty Reed. I was moved to tears many times by the accounts of her experience.