I can think of three main reasons why Gladwell includes his own family story as the epilogue.
(1) He has spent the entire book talking about other individual success stories and the elements that had to be in place for them to take place. Now he shows us that no one is immune to these kinds of “outlier” events, not even the journalist reporting on them. This makes the book more personal and more of a complete story in and of itself.
(2) By placing his story at the end, Gladwell shows us his humility. He talks about others first, then himself and his parents. This is a mark of a professional journalist. It’s not about him – at least, not at first. Also, by ending the book this way, he allows each one of us to think about our own backgrounds and our own cultural legacies. What were the unique circumstances that brought each one of us to life and to what we are doing now? Gladwell gives us a prod to think about this, even after we are finished reading the book.
(3) Because his story naturally prompts us to think of our own, it is wise for him to leave this for the end. What if he had started the book with his mother’s story of growing up in Jamaica? Then it would have seemed as though the book was mostly about the author, and not about these other interesting people and events. And since we are naturally prone to think of our own stories after reading his, we would be constantly thinking about ourselves while reading the rest of the book. And maybe we wouldn’t pay as close attention to the text as we should and otherwise would. No, Gladwell is a seasoned journalist and an accomplished writer. He knows all of the benefits of leaving his story for last. And he knows the lasting impact it will have on the reader, too.