Just as “cleanliness” must reside in deeds, so must it reside in traits. The work involved in changing one's traits is more difficult than that involved in changing one's deeds. This is because changing traits means doing battle with one's own nature.
Torah calls for preparation. The Patriarchs preceded the giving of the Torah, and, as the foundation of the Jewish people, fulfilled the entire Torah even before it was given. This measure was taken as a kind of preparation for receiving the Torah.