In this particular case I just happen to know exactly what they were thinking when they were implementing this feature, because they are my colleagues (even if I don't work on the team that works on C++) :) The list of features that they did was based on some specific libraries that they had most complaints about on Windows, and then filtered down further based on ease of implementation. If I remember correctly, one major beneficiary of those changes is supposed to be ffmpeg.
This all might make more sense if you remember that Office in some incarnation or the other now ships across three non-Windows platforms (OS X, iOS and Android), then there is the OneDrive client etc. Basically there's a whole bunch of stuff that has suddenly gone cross-plat in the past couple of years, and that's a lot of C++ code that now has to play ball with the libraries that are the de facto standard outside of the MS ecosystem. In many cases, once you start doing that, it makes sense to use the same library on Windows as well, but then you start running into those conformance issues with C99.
The other aspect is that we want people to write cross-platform C and C++ code, because it's the kind that, right now, is most easily portable between all mobile platforms - and seeing where Windows phones and tables are in terms of popularity relative to iOS and Android, MS has to encourage portability as a way to get more apps ported to Windows. You see things like Apache Cordova tools and Clang/LLDB support in VS 2015 for the same reason - they make it easier to write Android apps, for example, but more importantly, the way they encourage writing those apps just happens to be the one that emphasizes portable code. Now that is more geared towards C++, but the question of popular libraries written in C also comes up there.