Even OS X's C++ usage is kind of a historical oddity. OPENSTEP drivers were written in ObjC for an API called DriverKit, but OS X/Darwin/xnu replaced that with the C++ IOKit. The rest of the kernel is still C. I wrote a comment a few years ago explaining why I think that decision was made:
OS X and its descendents is the only exception.
Which goes back to NeXTSTEP using Objective-C and offering UNIX compatibility only as a path to bring software into the system, battling against SGI and Sun market space.