As we know when we write Hashem's name, we replace the hey with a kuf. Why davka a kuf? For example: Elokai, Elokeinu, Elokah, or "Yud-Key-Vav-Key". And in "Yud-key-vav-key", why is it pronounced that way instead of "Yud-kuf-vav-kuf"
R' Moshe Bamberger once said the the kuf (which means "monkey" in Hebrew) is a mocking imitation of the hei, since it looks similar.
This article takes this concept further, saying that while the hei represents the Divine kingship, the kuf represents the earthly kingship. However, in this world HaShem's presence is usually hidden behind a mask of the physical world, just as the hei is replaced with the kuf when pronouncing HaShem's name.