In the case of Pilgrim's Progress, I suspect it became a classic because it served as a blatant and approachable introduction to a certain school of Christian thought.
Autobiography of Benjamin Franklin Review
Don’t expect a text that exists primarily to inform or tell a coherent story, because that’s not what Franklin was setting out to do with his autobiography. It was instead intended originally for his son, and eventually for a wider audience of the burgeoning America, as a moral guide, an example and explication of how it might be possible to live a moral, productive, and well-regarded life, such as Franklin himself led.
IGC Publishing 