Moses did not need to enter the Holy of Holies, for he himself had a holiness of a higher level than that of the Temple.
(That is why Moses’ personal tent, where he sat and meditated, was given the same name as the sanctuary-the “tent of meeting.”)
In the essence of his being, Moses was the Holy of Holies from whose throat the Shekhinah spoke, just as it spoke from “between the two cherubs” in the physical Holy of Holies.
–Rabbi Adin Steinsaltz