If you look at recent open source projects, you'll see many of them use C++. KDE, for instance, has all of its subprojects in C++. But for projects that started a decade ago, it was a risky decision. C was way more standardized at the time, both formally and in practice (compiler implementations). Also C++ depends on a bigger runtime and lacked good libraries at that time. You know that personal preference plays a big role in such decision, and at that time the C workforce in UNIX/Linux projects was far bigger than C++, so the probability that the initial developer(s) for a new project were more comfortable with C was greater. Also, any project that needs to expose an API would do that in C (to avoid ABI problems), so that would be another argument to favor C.
And finally, before smart pointers became popular, it was much more dangerous to program in C++. You'd need more skilled programmers, and they would need to be overly cautions. Although C has the same problems, its simpler data structures are easier to debug using bounds checking tools/libraries.
Also consider that C++ is an option only for high-level code (desktop apps and the like). The kernel, drivers, etc. are not viable candidates for C++ development. C++ has too much "under the hood" behavior (constructor/destructor chains, virtual methods table, etc) and in such projects you need to be sure the resulting machine/assembly code won't have any surprises and doesn't depend on runtime library support to work.
