There is some information about this both in the Editor's Note to the book, and in Helen Keller's introduction to the first chapter. Keller explains that the thought of writing the history of her own life struck her as something different to other writing she had produced, and even filled her with fear, for several reasons. First, she had a sort of superstition about "lifting the veil" that lay over her childhood memories, not wanting to diminish any beautiful memory through the act of writing it down. Secondly, she had some difficulty in actually distinguishing fact from fiction in attempting to recall her childhood, and also felt sure that she had forgotten some important moments entirely.
In the Editor's Note, it is explained that Helen's recollections would not actually be able to present the full story of her life, because the nature of her education was so significant to the story as a whole. As such, the book is supplemented with reports and letters from Keller's teacher, Anne Sullivan, which explain more about how Helen Keller was educated and what Sullivan's system entailed.