I can think of several reasons.
There may not be a satisfactory C++ compiler. C++ is a much bigger language, and I've run C compilers on systems that would not be able to handle modern C++.
The questioner, or people he or she works with, may be familiar with C but not C++.
The project may be in C. While it's possible to add some C++ features to C, that can easily lead to an unmaintainable mess. I'd suggest picking one language or the other (usually C++, when practical).
The questioner may have an obsolete view of C++'s learning curve. (When approached correctly, it's easier than C's. Most introductory books I've seen don't approach it correctly.)
Remember that C and C++ are two different languages, and are getting more different over time. Coding in both at once is a bad idea, and using a C-like subset of C++ misses most of the advantages of C++.
