There is a second reason for the difference between Asiya and the other worlds. As known, all the worlds have ten sefirot [collectively]. Now Asiya, in its entirety, has only one sefira [of the collective ten], the sefira of malchut.
In the system of ten general sefirot, the World of Asiya corresponds to the sefira of malchut, whereas the World of Yetzira corresponds to six different and separate sefirot: chesed, gevura, tiferet, netzach, hod, and yesod. Thus, the World of Asiya is one homogeneous sefira, whereas Yetzira is a composite of six different sefirot.
Thus, the Nefesh that is there is able to rise as high as the keter of Asiya, because it is all one sefira.
In other words, even though Asiya has many levels, they are levels of one sefira, and therefore connected to each other, which allows movement amongst them.
The five partzufim in every world Arich Anpin, Abba,Imma, Zeir Anpin, and Nukva correspond to the five levels of a person’s soul
However, Yetzira corresponds to six sefirot: chesed, gevura, tiferet, netzach, hod, and yesod, each of which is a separate level. Therefore, if someone's root is the malchut of Yetzira and it becomes rectified, it cannot ascend and become part of the yesod of Yetzira. It must remain below and he will have to acquire a new Ruach from the yesod of Yetzira if he wants to become elevated through his actions. This is true of the rest of the "Six Extremities" as well.
In Hebrew, the term is "Sheish Kitzvot," the six extremities, another name for the six sefirot of Yetzira.
The five partzufim in every world - Arich Anpin, Abba, Imma, Zeir Anpin, and Nukva - correspond to the five levels of a person's soul, which are from the bottom to the top: Nefesh, Ruach, Neshama, chaya, and yechida.
Nefesh is from the Nukva [Malchut] of Zeir Anpin, whereas Ruach is from Zeir Anpin itself.
That is, the six sefirot: chesed, gevura, tiferet, netzach, hod, and yesod.
"Wisdom gives life to its owner"
Neshama is from Imma [Bina], and chaya is from Abba, which is called chochma because that is the place of life, as it has been taught concerning the verse, "Wisdom gives life to its owner". (Eccles. 7:12)
Yechida is from Arich Anpin, called keter, because it is alone and special…
The Hebrew word "Yechida" means both "alone" and "special". The partzuf of Arich Anpin is both alone and special relative to all the other partzufim.
It is alone and special with respect to the rest of the sefirot because it lacks a "female" counterpart.
Nukva is the female consort of Zeir Anpin. Abba-father has a female counterpart, Imma-Mother. Arich Anpin, however, does not have a counterpart.
This is known from the verse, "See now that I, I am He" (Deut. 32:39), as elucidated in the Zohar, in Parashat Bereishit.
This verse is spoken by G‑d, Who has no counterpart. Arich Anpin is also called "I". Thus, on the level of Arich Anpin, the highest level and closest to the One G‑d, the Infinite Creator, there is no counterpart and no corresponding female partzuf.
[Commentary by Shabtai Teicher.]