Wikitree uses many open source tools, but the wikitree code itself is not open source. The reasons cited are that they dont want competing versions and it would compromise privacy if it was open source.
The trouble with this approach is that the lack of any access to the source doesn't make wikitree a very enticing project for open source developers, there seems to be a distinct lack of developer activity for such a large and important project.
For example I'm working on a project myself to make it easier to create better wiki profiles for the people I've added to wikitree but I really wanted to be able to make my GEDimport work better with Wikitree Import but because the code is not available (and Ive asked) its impossible for me to do that part.
Security concerns are important but only really applies to a small part of the code base. Duplicate of sites is less of a concern, not least because any competitors you would need access to the full datadump and that is not readily available either.
So why not open things up a bit and get some help to make the project better.