For a while it had only blocked the Chinese version of the site, but now all versions have been put behind its firewall.

China has been blocking parts of the site, if not the whole site, on and off for more than a decade in the hope of preventing its people from learning things the government would rather they did not find out about.

It is believed (via Pixels Tech) that the root of this current block comes from Wikipedia changing its default protocol from HTTP to HTTPS, which has made it more difficult for China's GFW (Great Firewall) to filter specific pages and information. Because of this the government has now decided to block the entire site.