[bsder]

Because Microsoft basically won't support C11.

[pjmlp]

They do via clang, long part of Visual Studio installer (since 2015), Visual C++ as C++ compiler naturally not.

And they aren't the only ones, try to see how much of Apple or Google OS applications you manage to write with C11 alone.

[marcthe12]

They recently added it the roadmap though.

[pjmlp]

Where is this roadmap?

There is no public plan to add to Visual C++ anything more than what ISO C++ requires in terms of C source code and libraries compatibility.

Microsoft has contributed to clang on Windows for those that still want to use C.