It's an interesting question. One part of the the answer is that the idea is for there to be a single subtree off the root where "the operating system" lives.

It's an interesting question. One part of the the answer is that the idea is for there to be a single subtree off the root where "the operating system" lives. Where MacOS 10 has /System, OS/2 has \OS2, Windows has \Windows, and Digital Research DOS has \DOS, systemd Linux has /usr. Of course, another part is that this is an old idea, already implemented once in the commercial Unices a quarter of a century ago. They merged /bin into /usr/bin, with a symbolic link from the former to the latter.