It's not certain exactly why China decided to block other languages, but the move comes just weeks before the 30th anniversary of the Tiananmen Square protests, which culminated in a violent government crackdown. China typically blocks access to specific web pages referencing the tragedy, but it can't do that on Wikipedia after the site switched to HTTPS and made it effectively impossible for censors to see the individual pages people are viewing. A blanket ban would be the only way to prevent people from reading content, especially since translation is readily available.
The Chinese government hasn't been shy about clamping down on internet speech as of late, to the point where it will target Twitter users who criticize the government despite the social network being (officially) inaccessible in the country. The block on Wikipedia could nonetheless be more painful than some efforts. It cuts off access to a wealth of knowledge, particularly for non-residents used to an unrestricted site.