I write in C only when I need to; in my case I have a limited runtime environment available, so I can't use some other language (which I might otherwise prefer) with a larger runtime requirement, and I also already know C, but not C++. Also, none of the libraries I have to use have C++ APIs. Had I already known C++, and if there were C++ APIs available for the libraries I need, then sure, I might have used C++, but as it is, the results (a C++ program but full of C-style programming because of the APIs) would not be worth the effort of learning C++. C++ also imposes a cost; that of reducing possible contributors to those people who already know C++ – knowledge of C is simply more common than C++.
Learning C++ is easy. What's hard is being able to afford a computer that can compile it in a reasonable amount of time.
I've seen projects written in C / Assembly that build over 9,000 objects in about 20 seconds on just one PC, whereas more than a few C++ codebases I've seen in the past couple years, that can be about as long as it takes to build just one file.
I honestly suspect the only folks really writing C++ consistently these days, on things like LLVM, are the ones who work at huge companies, with literal supercomputers devoted to compiling their code, which would prevent them from ever encountering the latency issues that exclude poor folk such as myself from any hope of ever hacking on their codebase. It's sort of like that scene of Agent Smith talking to Neo in the interrogation room: "What good is the source code... if you're unable to build?"