C is a very important language, but not so much among Visual Studio's target market. Windows developers tend to be rather C++-centric, and I'm guessing the biggest reason why the compiler team hasn't bothered is because there just isn't sufficient demand to justify the diversion of resources.
I'm also guessing a lot of the world's C99 code has dependencies beyond the just the compiler that Visual Studio won't support without some effort. "Just compile it as C++" is not a good answer, but I wouldn't be surprised if "just install Cygwin" often is.