The reason is that per halacha you cannot do, on behalf of someone else, something that you yourself are not obligated to, and halacha says that women are not obligated in parts of the prayer service. (Some parts we are obligated in, and there are traditional communities that allow women to lead those parts.) So in that setting it would depend on what exactly is involved -- reading torah no, leading many prayers no, giving a d'var torah (speech) ok.