according to the dvd, DOTF is very "religious" in a sense. and thats because we have this big jedi religion thats still very much alive and even "in its prime" so to speak. you have the appearance of a threat to this established organization, which is its polar opposite. DOTF is perfect for such a dramatic conflict of good vs evil, and the dark brooding tone of the song almost suggests that evil will win. By ANH, the jedi are all but extinct. that conflct no longer exists, atleast in its grand form. It would be out of place in the OT. OT has more "hopeful" music.