what would prevent someone to make make a clone and use C++? Apart from the amount of work, of course.
Nothing. But upstream will not merge your changes and you will not be able to automatically merge changes from upstream, so you'll then be entirely on your own to maintain the codebase. Is C++ worth it?
Lets imagine a group of developers would create a kernel fork in C++. How would you get a distro, for example Ubuntu, to use it?
Presumably it would maintain the same ABI as Linux, so it should be a drop-in replacement for the kernel. It should just be a matter of modifying any of those distributions to load your kernel instead of Linux, and after that the system would run as if nothing was different.