Herb Sutter doesn't like C. That may be the reason that the compiler group at Microsoft don't put any effort into updating the C compiler to support newer features.
They do put a great deal of work into the C++ compiler, and seem to be doing a way, way, better job than they were in the late 90s.
That may be the other reason they don't update the C compiler with new features.
The reason is official and has been communicated many times.
C related improvements will only be done to the extent required by ANSI C++, or requests from key customers that might influence roadmap.
Anyone that really deeply wants to keep using C on Windows, and even enjoys using COM directly from C (the main ABI since Windows 7 and UWP core stack), can use any other C compiler.
In fact Microsoft has suggested clang multiple times, and has helped clang devs to make it work better on Windows.
They do put a great deal of work into the C++ compiler, and seem to be doing a way, way, better job than they were in the late 90s.
That may be the other reason they don't update the C compiler with new features.