Re: I don't like change
I think it was a matter of convention and knowledge. The install docs (V7 here, nroff source) for Bell Labs UNIX did not give very many hints about how to do it, and if all you did was to follow the docs with a single disk system, you would end up with a layout that probably left you with nowhere other than /usr to store user files (Sorry, I did have links to PDF formatted documents from the Lucent UNIX archive, but that appears to have disappeared - still, "groff -ms -T ascii filename" will make a reasonable attempt to format these for the screen).
On the first UNIX system I logged into in 1978 at Durham University, there was a separate /user filesystem which mapped to a complete RK05 disk pack (about 2.5MB per pack). / and /usr (and the swap partition) were on disk partitions on a separate RK05 disk pack. At this time in V6 and V7, disk partitions were compiled in to the disk driver (in the source), and IIRC, the default RK05 split was something like 25%, 60% and 15% for root, usr and swap.
Whilst I was there, the system admins. (mostly postgrad students) added a Plessey fixed disk that appeared as four RK05 disk packs, and allowed them to give ingres it's own filesystem. This happened at the same time that V7 was installed on the system, over Summer vacation in 1979.
When I installed my first UNIX system (1982, again V6 and later V7 UNIX), I kept a similar convention. although I had two 32MB CDC SMD disks to play with, configured as odd sized RP03 disks, and I split each of the disks up as either four quarter disks, 2 half disks or one complete disk - don't use overlapping disk partitions! (again in the device driver source of V7 UNIX). It was a very involved process getting UNIX onto these non-standard geometry disks, but that's a tale for another day.
During this time, I also had access to an Ultrix system which user /u01, /uo2 etc (BSD convention).
When I worked at AT&T (1986-1989), they also used the /u01, /u02... convention for user filesystems.
Following that, I've always had a /home filesystem for user files.