I can't speak for the elves, of course. But the reason my kind left is exactly that. We existed simply to save the world from the Dark. To protect humans from them and to ensure that they could choose their own futures free of any influence from the Dark. But once the Dark was gone we no longer had a purpose.

Oh, we could have stayed, like you said. We could have reveled ourselves to the world, and offered our advice and guidance.

But that is not our nature. The world belongs to humans, and it is their choice what its future must be. And whether it is for good or for ill, it must be their own choice, and they must take the consequences that come with it. It would be wrong for my kind to interfere with this.