— Every language out there has a way to call into C
— If your code is slower than C, someone will rewrite it in C.
— If your library is written in C it means it can be used on any OS, console or mobile device and even on the web.
— Not everyone wants to use C++ (some prefer C).
— It is easier in general for a C++ user to use a C library than it is for a C user to use a C++ library.
— C++ is not as easy to write wrappers for in other languages.
— Unless you limit which C++ features you use (to the point where you are pretty much left with C) a lot of people won’t be able to use your library.
PS. If your original comment had been phrased the way you put it here I might have made the same comment, but I would not have downvoted it. Here, you’re at least providing some supporting evidence for your assertions which makes it a much more valuable contribution to the conversation.