We can only speculate on the answer to this question, as Ms. Lee has traditionally been a very private person and has never given a public answer to this question. However, we can examine what we know about her autobiography and see many similarities between her own life experiences and those of Scout's.
To begin, Ms. Lee was born in 1926 and grew up in Monroeville, Alabama. This setting is almost synonymous with that of the novel--the midst of the Depression in fictional Maycomb, Alabama. Ms. Lee's mother's name was Frances Cunningham Finch Lee; three of these names are borrowed for characters in the book. Ms. Lee's father was both a lawyer and a politician, mirroring Atticus Finch. Truman Capote (an acclaimed author himself) was Ms. Lee's childhood friend and summertime neighbor and the obvious model for Dill Harris.
Perhaps, though, despite all these obvious similarities between her life and her novel, Ms. Lee wanted to send a message about the injustices she saw in the world around her. The book was published during the Civil Rights era, and it speaks to so many issues that were still prevalent in that turbulent time. Perhaps Ms. Lee wanted to make a statement about the wrongs she saw that people would actually hear through all the noise, so she chose the voice of an unassuming, charming little girl from an equally unassuming, charming little town. Perhaps one day Ms. Lee might just decide to tell us herself.