It was a historical quirk to start with. At Bell Labs, back in the early 1970s, Unix was being developed on PDP-11s with RK05 hard disks (with removable disk packs), which had an amazingly generous capacity of 2.5MB each. The Unix operating system had grown too big to fit on a single RK05 disk volume so they had to split it across two. Other operating systems of the period faced similar issues, but dealt with them in (arguably) more elegant ways – on IBM mainframes, OS/360 maintained a database ("catalog") mapping file paths (dataset names, to use the proper terminology) to volume names, so you could move a file to another disk without changing its path. True to Unix's penchant for simplicity, its authors decided instead to just split the OS into / and /usr. And the split survived long after they'd upgraded to more spacious disks.
Any other explanation for the split is essentially a retcon. Some of those retcons (even if, as other commenters have pointed out, not your own) may actually have become true – some of them may have been approximately true to begin with, and they influenced people's decisions, thereby making themselves more true over time. But its ultimate origins will forever remain this quirk of computing history.
I don't mean to shame you, I sometimes comment without reading TFA, and in your case you add a few more details that were not present in the article. I just found it interesting.