The social aspects are very large and it wouldn't surprise me at all if that was by far the main reason... but that's much less of an issue in a kernel-like con

The social aspects are very large and it wouldn't surprise me at all if that was by far the main reason... but that's much less of an issue in a kernel-like context where there are already oodles of rules beyond "write valid C code". They can and do impose significant limitations on the languages they use, successfully, for decades. Seems like they'd be able to do that with C++ too.