Why do operating systems do low level stuff in C and C++? Why not just C++?

Question

On the Wikipedia page for Windows, it states the Windows is written in Assembly for the bootloader and task switcher, and C and C++ for kernel routines.

IIRC, you can call C++ functions from an extern "C"'d block. I can get using C for the kernel functions so pure C apps can use them (like printf and such), but if they can just be wrapped in an extern "C " block, then why code in C?