A not inconsiderable part of why is that Rust for Linux did the work.

When C++ people say they think there should be C++ in Linux, their proposal usually begins by proposing that it "should" be possible to just compile Linux as C++ software. This doesn't work because C isn't just "C++ but old", and they rapidly lose interest.

Which of course also feeds into Linus' semi-fair claim that not allowing C++ keeps out the low effort wannabes who would plague such a project. This makes C++ developers very angry, but part of the reason why is that it's true, C++ does attract these people.

The Rust for Linux people wrote a lot of code, a lot of documentation, they did Q&As, they worked very hard to actually deliver the idea to the kernel community, it's a totally different approach, it's a lot more work but some people thought it was worth the work.