Because the root directory might be on a very small partition (perhaps only a few hundred megabytes), while other mount points like /usr might have more space;

Because the root directory might be on a very small partition (perhaps only a few hundred megabytes), while other mount points like /usr might have more space; the only things which should be in / are the things which are necessary to mount the other filesystems (perhaps through the network using NFS). (Yes, nowadays hard disks are much larger, we have things like initrd, and we now make /bin and /sbin symlinks to within /usr, but the parent comment did mention the 90s...)