The kaddish mentions the Almighty but I would have thought it would have also mentioned the departed. What is the reason for this?
Chabad.org gives the normative view of kaddish (my emphasis)
While Kaddish is commonly known as the "mourners prayer," a reading of the text reveals that it is not about death or mourning, but the public proclamation of G-d's greatness. By rising from the depths of anguish and loss to offer praise to G-d, we transform the event of death into an act of life.
So because it is not about death or mourning, the name of the deceased is not relevant. It does however bring merit to the deceased in the World of Truth. See for example the story about Rabbi Akiva and the Orphan.