C is so tiny that if you're going to write anything non-trivial you either have to re-implement various common data-structures yourself, or find appropriate libraries. No built-in string type, no vectors, no nothing. Also, the lack of generics makes for really ugly code, filled with void pointers and casts whenever you attempt to write type agnostic data structures such as lists, queues, etc.
It also seems that whenever I have to write in C, I end up writing code that painfully wants to be object-oriented, or implementing features that are already part of C++.
So why would you willfully pick to use C when C++ is around? Heck, even if some sick reason you don't want to use explicitly use objects, at least you can enjoy some of the other benefits of C++ (e.g. its standard library).