Here is that "mental and moral" argument referred to in The Rhetorical Tradition introduction to this section.

I think this is key. Women were held to exacting (actually, impossible) standards regarding morality, while completely disregarded intellectually. However, restrictions on women did not stop there, but continued into a swirling confusion of contradictory stereotypes:, for example, that women are naturally inclined to corrupt men through seduction and lasciviousness, yet are also naturally innocent, naive and in need of protection. Some early feminists mistakenly began their argument for women's rights with the premise that women deserved recognition for their moral authority, while giving in to accusations that they were not as intellectually capable. This naturally left any of their arguments suspect, as they were admitting (even if only for the sake of humility) that they might not be able to match the arguments of men intellectually. By positing that men and women are equal in terms of BOTH intellect and morality, Grimke builds herself a more sturdy rhetorical platform.