C is effectively frozen in time, thanks largely to Microsoft's decision to deprioritize C support in favor of C++. C99 is almost 20 years old and MSVC still doesn't fully support it. Forget about C11. If you're a C programmer who cares about portability, you stick with C89. There's not a lot of point to having a conference about something that is stuck on a 30 year old standard.
MSVC is becoming less and less relevant. People are writing C99 and C11 and MSVC's insistence on breaking portability with modern codebases is incrementally marginalizing it as people switch to compilers that understand their standard compliant code.
It is called MSVC++ for a reason.