A couple of reasons might be:
- Lack of support - Not every C compiler is also a C++ compiler. Not all compilers are particularly compliant with the standard, even if they claim to support C++. And some C++ compilers generate hopelessly bloated and inefficient code. Some compilers have terrible implementations of the standard library. Kernel-mode development generally makes use of the C++ standard library impossible, as well as some language features. You can still write C++ code if you stick to the core of the language, but then it may be simpler to switch to C.
- Familiarity. C++ is a complex language. It's easier to teach someone C than C++, and it's easier to find a good C programmer than a good C++ programmer. (keyword here is "good". There are plenty of C++ programmers, but most of them have not learned the language properly)
- Learning curve - As above, teaching someone C++ is a huge task. If you're writing an app that has to be maintained by others in the future, and these others may not be C++ programmers, writing it in C makes it a lot easier to get to grips with.
I'd still prefer writing in C++ when I can get away with it, and overall, I think the benefits outweigh the disadvantages. But I can also see the argument for using C in some cases.
