I think there’s more to it than the legacy of Romanticism, in which writers loved to torture the desirable but disposable courtesan and prostitute (Splendeurs et misères des courtisanes, La Dame aux Camélias, Nana, etc.) and their spin-offs (La Traviata and so on).
I recently read a very good historical novel, Beautiful Lies by Clare Clark. As wonderful as it is, when compared to Clark’s The Great Stink, whose chief protagonist is male, or The Nature of Monsters, whose protagonist is a pregnant teenager, Beautiful Lies seems to have less pace and tension. I feel that in a Victorian-era setting, having a respectable (or seemingly respectable) female protagonist is a disadvantage.
One of the limitations of writing about a middle or upper class woman is that women’s lives and movements were curtailed (they could not go out unaccompanied, they could not participate in that activity, they could not reveal those qualities, they would have been kept in ignorance about that topic, they could not enter those places). Women had to do a lot of waiting and watching, unable to take the reins themselves (advantageous for the early detective stories, but not convenient for all novels).
Poorer women did not have the restriction/protection of chaperones but their lives were trammelled by lack of money, education and opportunities. They traced and retraced the small circuit between paid work and family work, experiencing little beyond hard graft, often dying close to their place of birth. Early on, I ditched the idea of having a woman as the chief protagonist for one of my own works in progress (a novel based on real Victorians), because I haven’t found a way to turn the limitations to dramatic advantage, even with a third-person narrator. It sucks the energy out of a novel when the protagonist, Hamlet-like, cannot take action.
I think this is one of the reasons historical novelists often write about prostitutes and fallen women: they venture further afield unchaperoned and thanks to their clientele, have a wider circle of acquaintance that crosses class boundaries. They are also, of course, outsiders and witnesses to society’s hypocrisy, able to comment on the great gap between what people proclaim and what they do. Their lives lend themselves to dramatisation as their very existence elicits shame, disgust, indignation, desire and denial.
Having looked into the mirror of a fallen woman’s life, society recoils or lashes out. These are the tensions exploited by novels such as The Crimson Petal and the White, Slammerkin, The Observations, and The Dress Lodger. The lives of respectable women (or women trying to pass themselves off as respectable) could be filled with private drama but they had fewer opportunities to upset the apple cart. A fallen woman, on the other hand, is a lightning rod. As an outcast, she is often outspoken, frank and willing to take a risk or have a laugh. Why play by the rules when you’ve already been cheated?