C99 is still new! Microsoft tried to kill C by refusing to implement anything that wasn't also in C++. MSVC was 16 years late implementing C99, and implemented only the bare minimum. Their C11 implementation is only 11 years late.

I suspect that decades of C being effectively frozen have caused the userbase to self-select to people who like C exactly the way it is (was), and don't mind supporting ancient junk compilers.

Everyone who lost patience, or wanted a 21st century language, has left for C++/Rust/Zig or something else.