First some background:
The original Pi fell uncomfortably between two stools hardware wise. A debian "armel" userland (with a Pi specific kernel) could run on the Pi but was far from taking advantage of it. Debian "armhf" wouldn't run because it's minimum CPU requirements were too high. To get around this Mike and I formed the Raspbian project and set about re-building all of Debian and I have been maintaining Raspbian since.
While we did produce one or two complete OS images in the early days, the Raspbian project has mostly focused on maintaining a repository of Packages and left the building of OS images up to other people. Some time later the Raspberry Pi foundation started building their own Raspbian images.
Over the years the delta between plain Raspbian and the Raspberry Pi foundation Raspbian images has grown as Raspberry Pi have developed their own desktop environment and have backported a substantial number of graphics related packages in support of their migration from their Pi-specific graphics stack to a Mesa based graphics stack. I have not been particularly happy with the lack of distinction between plain Raspbian and the Raspberry Pi foundation Raspbian images but I also didn't feel like pressing the issue too hard.
Separately the Pi lineup has been evolving. The original Pi was using ARMv6 CPU, the Pi 2 was using ARMv7. It could run a Debian "armhf" userland and after a while Debian also added support for the Pi 2 in their kernel, though being an "upstream" kernel some things that are supported in the downstream raspberry pi kernels are not supported. The Pi 3 added 64-bit cores, which (after a bit of kernel development) meant Debian "arm64" could now run on the Pi. Then the Pi 4 came along offering up to 4GB of RAM.
Through most of this the Raspberry Pi foundation decided to stick with a single OS image based on Raspbian as their official main OS. They decided that the benefits from multiple OS images did not justify the extra work.
So that brings us forward to April 2020. The 8GB Pi 4 was in alpha testing and Raspberry Pi decided it was finally time to start producing a 64-bit OS image. I got an e-mail from Eben asking my opinion on naming. I expressed that I would not be happy about the name Raspbian being used for an image that did not actually use anything from the Raspbian project. The name Debian wasn't exactly great either because Debian were building their own images for the Pi.
So Raspberry Pi decided to use the term "Raspberry Pi OS" for all their OS images (32-bit for Pi, 64-bit for Pi and 32-bit for PC) based on Debian or Raspbian.