I realize it's neither a professional nor a particular good answer, but for me it's simply because I really like C. C is small and simple and I can fit the whole language in my brain, C++ to me has always seemed like a huge sprawling mess with all kinds of layers I have a hard time grokking. Due to this I find that whenever I write C++ I end up spending far more time debugging and banging my head against hard surfaces than when I code C. Again I realize that a lot of this is largely a result of my own 'ignorance'.
If I get to choose I'll write all the high level stuff like the interface and database interaction in python (or possibly C#) and all the stuff that has to be fast in C. To me that gives me the best of all worlds. Writing everything in C++ feels like getting the worst of all worlds.
Edit:
I'd like to add that I think C with a few C++ features is largely a bad idea if you're going to be several people working on a project or if maintainability is priority. There will be disagreement as to what constitutes a 'a few' and which bits should be done in C and which bits in C++ leading eventually to a very schizophrenic codebase.
