Existing C code needs to be maintained, and can take advantage of the newer features when available in the compiler. The Linux kernel is moving to C11, and may move to C17/C23 later. Also not everyone wants to put up with the compilation times, object sizes, and aesthetics of Rust.
I doubt that the kernel will adopt the C++ memory model (the big change in C11). Instead, they will keep doing their own thing. Given the problems with the memory model, I can't really fault them. But framing this in terms of standards versions is a bit of a stretch. They could easily adopt additional GCC extensions over time as they move minimum compiler versions forward. Standardization does not really matter there.
As for new developments, see for example https://news.ycombinator.com/item?id=33675462 which uses C11.