I always thought of it along historic segregation of CW and Phone, that only recently has begun to adjust from the 50/50 split When Time Began.
Actually it started out not even a 50/50 split. The original "80-metre" phone band was only 50 kc/s wide and located at the low end, 3500-3550 I believe. Then, in 1932 or '33 it was changed to 3900-4000. By the time I was licensed in 1959, it had been expanded to 3800-4000. I seem to recall that for a while after the War it was 3850-4000.
There was no phone on 40m before the War. On 20, the phone band was located smack in the middle. The idea was to keep sideband splatter contained within the ham band to avoid QRM to the prime international telecommunications frequencies used by commercial and government stations in those pre-satellite days. I don't recall the exact history, but by the mid 30's it ran 14150-14250. The entire ham band used to be 14000-14400, so this left 150 kc/s of cw "guard band" on each side. In the early 30's 10m phone was only a small portion, 28000-28050 (or was it 28000-28500?). The entire band was 28000-30000 in those days.
160 was also more restricted at one time. There is a notice in one of my 1933
R/9 magazines announcing that 160m phone had been
expanded to 1800-2000. The entire band ran from 1715 to 2000. We never got the 1715-1800 segment back after the War. We are lucky we got any of the band back. Right after the War there was fierce opposition even to our shared status with LORAN.