The crux of the issue I believe is C++ can lead a bewildering mess of coding styles and abstractions, making it difficult for the thousands of people working on the kernel to grasp each other's code. This would be mitigated to a large extent by using the simpler C, hence the rule.

C++ needs a lot more discipline and thought if you don't want unmaintainable code years down the line. Ironic given that abstractions introduced by C++ are partly to make code more efficient to write, extend and maintain.