There could be several potential answers to such a question. I would say that the mere title could be a good indication of the primary motivation of the author. Tituba gets a fairly rough rap from a historical point of view. She is one of the first to be thrown under the proverbial bus by the girls of Salem, and the townspeople have little problem in being able to persecute her as the cause of all that is wrong in Salem. Even when the townspeople realize their error in judgment, they do not offer any salvation for Tituba, almost as if the miscarriage of justice that happens to her is not that as relevant. The historical fiction humanizes her and brings out a potential read of her voice as a woman of color, two elements that would have rendered her fairly silent through the time period. To give voice to a character that lacked it might have been one reason for the book's composition.