When doing low level system programming, there aren't that many viable choices out there. C, C++, possibly ObjectiveC (not familiar enough with it to tell for sure). That's about it. Of those, ObjectiveC is, pretty much, a one platform language. C++ is used quite extensively, but it is way too complex, resulting in most C++ programmers not knowing what the 1@#$@!# they are doing. Also, some C++ features are not suitable for some low level scenarios. For example, you probably wouldn't want your kernel code to throw exceptions, or do iostream formatting, in kernel code.
C, on the other hand, is a very simple language. It has no expensive features (though, to be honest, that mostly means that if you need something expensive, you'll need to do it yourself). As such, it is without competition for what it offers. The most it loses in market/mind share is through scenarios that used to require low level system programming but no longer do.
As for D....
D advertises itself as supporting this mode. My employer chose to develop a low-level high performance low latency system in D. I've been programming it for the past half year. I'm not overjoyed. I don't hate D, but my personal opinion is that we'de have been better off going with C++ (though, to be honest, I love C++ like few of my peers do).
I have two main gripes with it on that front. D has a horrid GC (though no GC provides the latency requirements we need), and though it claims you can do without it, you really can't. At least, not without giving up on much of the language features and almost all of the standard library. When comparing to C++'s ability to use custom allocators with the standard library, D's phobos seems deathly pale.
D also claims to support RAII semantics. I happilly went about implementing a reference counting pointer, only to find out that there are cases where you cannot use a struct with a destructor, and there are cases where you theoreticaly can use one, but in practice find that the compiler will not call your destructor. All in all, RAII is an untested unutilized option in D.