Except for 4e of course.I think it comes down to the role or purpose of angels. The nature of celestials and fiends are vague and leave it up to DMs.
Are they servants of gods? Do they fall when they stop serving? What about evil gods, do they serve them? Or do evil gods have devil's or demons or something else? Would chaotic gods use reclaimed devils?
D&D doesn't focus that much on the combination of polytheistic and monotheistic aspects it has. It makes most powerful celestials and fiends into alignment robots.