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That is only because on Windows, C is seen as legacy language, and Microsoft has decided to focus its development effort on C++ instead.
Granted, this is typical Microsoft only caring about its own stuff, but in many IT shops, C is only used when it is not possible to use C++.
C is hardly a legacy language on Windows; the Windows API is still all C.
Since Vista, most of the new APIs are actually COM based.
Which although you can use from C, there is only proper compiler support when compiling from C++.
True, but MS is trying to push users into using C# instead of C/C++.
In the long run, I would imagine that MS will try to kill the C Win32I (and NT) APIs and drop support for "legacy" C/C++ applications.