The defining quality of the Beis HaMikdash is that the Temple site itself becomes sanctified. Therefore, the distinctions between the various levels of holiness within the Beis HaMikdash were expressed in the physical elevation of the place. Accordingly, one had to ascend steps when passing from one level of holiness to another within the Temple itself.
The Rambam states in Hilchos Beis HaBechirah, chapter 1:
“The entire Temple was not situated on level ground, but ascended along the mountain. When a person entered… he would proceed on level ground until the end of the Cheil, ascend from the Cheil to the Women’s Courtyard… ascend from there to the Courtyard of the Israelites… and proceed across the entire Courtyard of the Israelites on level ground…”
The Mishkan, by contrast, was entirely level.
This explains why the Rambam emphasizes the negative wording:
“The entire Temple was not situated on level ground.”
He does not suffice with stating positively that the Temple ascended along the mountain. His wording is intended to exclude the possibility that the Beis HaMikdash resembled the Mishkan in this respect.
The reason for this distinction between the Mishkan and the Beis HaMikdash requires explanation. Seemingly, even though the Temple stood upon a mountain, it could have been constructed on one level, just as the Ulam and the Heichal stood upon the same level.
The explanation is as follows:
The holiness of the Mishkan derived from the sacred components of the Mishkan, but not from the physical site itself. Since the Mishkan was a temporary structure, its different degrees of holiness were not expressed through permanent distinctions in the physical ground.
The Beis HaMikdash, however, is defined by the fact that the place itself becomes sanctified. Its sanctity is so permanent that:
“Sacrifices may be offered even though the Temple building is no longer standing.”
Therefore, the distinctions between the various levels of holiness within the Beis HaMikdash were expressed in the physical place itself. One consequently had to ascend steps when proceeding from one level of holiness to a higher level within the Temple.
Nevertheless, there were no steps between the Kodesh and the Kodesh HaKodashim.
This is because the holiness of the Kodesh HaKodashim is incomparably higher than the holiness of the Kodesh. Such an immeasurable distinction cannot be expressed by ascending a specific number of physical steps.
There is also a deeper explanation.
Regarding the other ascents, the Rambam emphasizes that a person enters, walks, and ascends. These movements are connected with the person’s own existence and his Divine service.
The Kodesh HaKodashim, however, corresponds to the level of yechidah, the innermost and singular essence of the soul. At that level, the person’s independent identity is completely nullified.