[Rough Translation] There is another reason that I will explain regarding the difference between a part of the soul a person has in Asiyah, and the part of the soul he has in all of the other worlds. And this explanation is, that we know that all of the other worlds [Yetzirah, Beriah, Atzilus] are comprised of ten sefiros, but the entire Asiyah is only comprised of one Sefirah, and that is the Sefira of Malchut.
Therefore, the Nefesh which is there, it is able to elevate itself all the way to 'Keter of Asiyah' because all of Asiyah is only one Sefirah. However, [in the greater picture], Yetzirah [which comprises the emotional attributes of an Adam] comprises six Sefirot -- Chesed, Gevurah, Tiferet, Netzach, Hod, and Yesod, as we know. And each one of these is separate one from the other. Thus, one who has the source of his soul in "Malchut of Yetzirah," even though he perfects it, he is not able to elevate that soul and complete it and raise it all the way to the higher levels of Yetzirah; rather, it must stay below [in its place], and the Adam (person) acquires a new Ruach from "Yesod of Yetzirah" if he wants to elevate himself to that level through his good deeds (Torah and Mitzvos). So too concerning all six levels of Yetzirah, as described.