Survivor Bias.
If some other random Encyclopedia had survived and not Wikipedia, you'd be asking "Why did <other random Encyclopedia> succeed while other encyclopedias failed?"
There's nothing really special about Wikipedia except that it survived while other websites didn't. There's only "room" for one or two encyclopedias on the Web, it's very possible that some other encyclopedia is destined to replace Wikipedia in the future, just like FaceBook replaced MySpace.
