The ease of use of Subversion is down to it being a centralized version control, which is pretty easy to understand. Switching to a distributed version control like Git takes quite a bit of effort (which I believe is worth it for more complex code bases being maintained by multiple people).

However, for a lot of business documents you want a single current version, with a history - Google Docs, Office 365 and similar give that to people in more user-friendly way than either Git or Subversion does, which is why they use it.