The main reason I think was that C++ support in open source compilers used to be rubbish, and the libstdc++ used to break binary compatibility all the time (from what I've heard).
That was many years ago, but people still think C++ is rubbish because of this, and it failed to gain traction in the open source community because of it. Which is a shame, because it's quite a nice language once you learn the details and are able to use it properly.
I think it was language religion above anything else, but my memory is a bit fuzzy from those discussions.
Personally I have a sweet spot for the Pascal family of languages and their strong typing, so I only started using C and later C++ when forced to do so.
My first contact with C was at high school (Turbo C 2.0 - 1992), after knowing already a few Assembly versions and Turbo Pascal. Shortly thereafter I got to have my hands on Turbo C++ 1.0, and since then only used C instead of C++ at the university classes that required it, or on a few projects at work. My last application done in C was around 2002.
Nowadays I use a lot of languages depending on the project, but in what concerns C and C++, I will always pick C++ unless requested otherwise.
That was many years ago, but people still think C++ is rubbish because of this, and it failed to gain traction in the open source community because of it. Which is a shame, because it's quite a nice language once you learn the details and are able to use it properly.