Tom (the original author) wrote about that a bit: http://www.arguingwithalgorithms.com/posts/14-01-03-content-.... In general I think we want to do a lot of custom things, so we'd need a very deep integration with an existing CMS, to the point where the costs would probably outweigh the benefits. For example, we have our own editor for articles and exercises (https://github.com/Khan/perseus), which does lots of fancy things, so having a prebuilt editor wouldn't be useful, and in fact would be something we'd have to replace. Controlling the whole system is just simpler for us.