Why Us?

So, in the last D&D session, it became established that the world is in need of some saving. One of the PCs raised the quite reasonable question, "Why don't these other, more potent folks go save the world instead of us?"

The OOC answer is obvious: this is a story about the PCs, and it's cooler to have the PCs save the world. But I'm making a mental inventory of possible reasons why these PCs (who are not yet the most powerful heroes in the land) might end up saving the world:

- The PCs are the only ones who realize the world needs saving.
- The PCs have special knowledge that no one else has.
- The PCs are in the right place at the right time, and have the opportunity to change the course of events.
- The PCs have a special item that is seems minor but becomes important.
- Ditto, but with a special ability or quality.
- The most powerful folks are engaged in direct action, while the PCs get sent on an indirect errand that swings the whole course of events.
- The most powerful folks all get killed.
- The PCs are considered more expendable than the more powerful folks.
- The more powerful folks are frauds who are not so powerful after all.

Hmm, that's only nine, and I figured that I could get up to a dozen at least. What am I missing?