Take this as my own personal opinion, but before I went back into engineering, I worked with a bunch of developers out there to integrate with all kinds of our stuff - and outside of game development, C/C++ isn't the most popular choice. We have some code samples [0], but I see what you mean.
The office team moving away from the old plugin model and opening up the gates for JS apps and REST APIs made life dramatically easier for a lot of folks. So I assume that samples were written with the most popular choices in mind. Internally, C++ is king [0], of course.
It's also worth pointing out that we always accept code samples from the community :)
[0]: https://code.msdn.microsoft.com/office/site/search?f%5B0%5D....
[1]: http://cppcon.org/bonus-talk-cxx-in-ms-office-2014/