For one, it is horribly slow. I do not know for a fact that its slowness is caused by C++ as a language or by the quality the programmers who wrote Monotone... but it is slow nevertheless and that is all I know. By contrast, git is extremely fast. Go figure.
John Carmack is a C programmer coding in C++ which fits pretty well with sirclueless's point about that it is the C++ programmers that are the problem. All his games before Doom 3 were implemented in C.
I imagine he had some help with that. And a lot of what he's been talking about recently is about tools for keeping code quality high; he obviously places a premium on good code over working code.
My point is, you can write good, readable, performance, large-scale codebases in C++. Note that Carmack wrote his previous engines such as Quake3 in C, and switched to C++ at some point. I think one of the reasons was that as their programming team size grew from 5 to 50 at id, C++ started making more sense for them. If you look at a book like "Large Scale C++ Software Design", the subset of C++ it uses is about the subset that I think makes sense in practice, roughly C with classes. To be fair, I think that was the language subset that actually worked in compilers at that time.
Irrespective, I don't doubt that C is a valid and perhaps better choice for writing an OS kernel. There's less abstraction in C, and you have to worry less about making sure you don't use a language feature of C++ that has a hidden cost.