Familiarity, and all the libraries and tools available for C++. I see that D has a section on C++ interop,[0] but it looks about as painful as FFI usually is, and even more painful given how template-heavy C++ code tends to be.
(Completely unrelated: I can't mention FFI without also mentioning how amazing LuaJIT's C FFI is. The developer(s) really nailed it.)