It seems to me it may just be that maintaining an open source project might be a little harder than maintaining something that is closed source simply because the maintainers can maintain it at their leisure, rather than having to deal with what comes with having a large open source project. 4chan is probably enough of a time sink for moot as it is.

Personally I believe 4chan seems like a good candidate for the MEAN stack. A lot of the features and boards could be coded as modules and it seems like it would be a lot easier to maintain, but of course this is mostly speculation. What might be cool is if 4chan/moot sanctioned an opensource project (or had a competition) to see if users could create a good open source "foundation" for 4chan and other image boards, then open sourced what was built off of that. I know 4chan has been "re-written" but I imagine some of the code is still original and I don't think any one expects it to be "professional quality" simply because it was written by someone who was (probably) completely new to web coding, so I can see why it may not make since to open source what they have already because it could lead to more bugs than it fixes.