If you work for a while at a larger company that enforces a style guide, it starts to make total sense. If your company's style guide already insists that identifiers should be named with some convention that matches their visibility, then by making that part of the language, you are simplifying the system.
I would not be surprised if specifically the Google C++ style guide influenced this decision.
I would not be surprised if specifically the Google C++ style guide influenced this decision.