Now we have already C11, and AFAIK there isn't a single compiler that fully supports C99, let alone C11.
Then we have companies like Microsoft which nowadays care only about C++, and won't improve the C standard support beyond C89 on their compiler.
http://gcc.gnu.org/c99status.html
Plus not all projects are allowed to use gcc or clang.
Case in point, I worked in a project where the UNIX compilers used had to be the standard vendor ones for Aix, HP-UX and Solaris.
Now we have already C11, and AFAIK there isn't a single compiler that fully supports C99, let alone C11.
Then we have companies like Microsoft which nowadays care only about C++, and won't improve the C standard support beyond C89 on their compiler.