Why are drivers and firmwares almost always written in C or ASM and not C++?

Because, most of the time, the operating system (or a "run-time library") provides the stdlib functionality required by C++.

In C and ASM you can create bare executables, which contain no external dependencies.

However, since windows does support the C++ stdlib, most Windows drivers are written in (a limited subset of) C++.

Also when firmware is written ASM it is usually because either (A) the platform it is executing on does not have a C++ compiler or (B) there are extreme speed or size constraints.

Note that (B) hasn't generally been an issue since the early 2000's.

Code in the kernel runs in a very different environment than in user space. There is no process separation, so errors are a lot harder to recover from; exceptions are pretty much out of the question. There are different memory allocators, so it can be harder to get new and delete to work properly in a kernel context. There is less of the standard library available, making it a lot harder to use a language like C++ effectively.

Windows allows the use of a very limited subset of C++ in kernel drivers; essentially, those things which could be trivially translated to C, such as variable declarations in places besides the beginning of blocks. They recommend against use of new and delete, and do not have support for RTTI or most of the C++ standard library.

Mac OS X use I/O Kit, which is a framework based on a limited subset of C++, though as far as I can tell more complete than that allowed on Windows. It is essentially C++ without exceptions and RTTI.

Most Unix-like operating systems (Linux, the BSDs) are written in C, and I think that no one has ever really seen the benefit of adding C++ support to the kernel, given that C++ in the kernel is generally so limited.