The reason is that by restricting yourself to gcc and clang, and agreeing how to define some undefined parts of C with those compiler authors, the amount of assembly you need drops massively, which is particularly useful when you are trying to support as many architectures as Linux.

While technically this means your kernel is "gcc c" rather than "true c", this doesn't cause a practical problem. I know gcc, clang and visual studio's c++ standard libraries require a flat address space and 8 bit bytes, for example, but I've never seen anyone bothered by this.