Let’s see. Janie is the character whose life we follow throughout the book. She is also the character whose thoughts and feelings we are best acquainted with through her own speech and through the narrator. That makes us like her because we understand rationale for her actions. She’s also the obvious underdog, victimized by situations and men she cannot control. We sympathize with her hardships because we see how racism and sexism work against her, through no fault of her own. And many readers can relate to her quest for true love, having gone through a starry-eyed adolescence of their own. All these qualities of hers pretty much make her the obvious protagonist.