I wonder why nobody has forked Wikipedia yet. There's no restriction to doing so, is there? To avoid having super high server costs, it could even be a "soft-fork", where only the pages which are edited are stored locally, all other content is fetched from the original.

If these restrictions are really affecting the quality of the articles in Wikipedia, we'll see the quality of the articles on the fork grow, at which point they can be merged back by Wikipedia.