Since this is a work of fiction (albeit based on the story of a real character), the only way we could definitively find out why Maryse Conde decides to have Tituba leave her home to be with John Indian would be to ask her. Since this is not practical, I would argue that the reason Tituba left her home to be with John was love. While I do not believe he deserved her love, because he later abandoned her, I surmise that Tituba, who had been alone a long time, relished the idea of companionship.
During her days of living in the forest, when she only had the spirits of her departed mother and Mama Yaya for company, I would imagine that Tituba became extremely lonely. After she meets John, these spirits try to convince her to stay in the forest, and this path could definitely have saved her some trouble. However, love causes Tituba to follow John.
In addition, Conde needed Tituba to leave the forest so that her story would progress and she would experience life in Salem. It is here that she would become both famous and infamous for her healing powers. Ultimately, Tituba is freed by Benjamin, who makes it possible for her to return to Barbados. Here, she ultimately returns to her old home in the forest, but she is older and wiser than when she left.