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The reason there are so few angels and demons in D&D is that they are both religious. For a very long time thanks to the RPG = debil worship panic, those entities were not part of D&D and had their serial numbers filed off. Anyone interested in playing an angel may not bother to look up archons or devas. Angels also suffer from the major problem with ghosts.
Every culture on the planet has an idea of what is a ghost. Many of these are completely different. Ghosts tend to work best when they're mysterious. They do things that "break the rules". It's very hard to make balanced mechanics for that while still preserving that mystery. They also have the problem of being very RP-centric in their threat and groups that prefer a hack-and-slash approach will have problems seeing a ghost as a threat.
If you like the idea of using ghosts more frequently, you'll want to look into the Ravenloft campaign setting and the Ghostwalk campaign setting. If you play 4th Edition, look at Hammerfast. Your best resource for writing adventures is actually things like creepypastas and old ghost stories. The information's out there, but they don't get the focus that drow, vampires, orcs, etc. get because those races can be used in both RP-heavy and hack-and-slash campaigns effectively.
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