Because it was not always the case that sysvinit was supported - things were sorta “accidentally hazy” for a while. There was a time (I think during Debian 9 and 10) that systemd not only was the default, but was also enforcedly linked against a large part of the stack (you couldn’t have a desktop environment, PulseAudio or NetworkManager without systemd, for example).

This led to the rise of projects like Devuan, that provide a working system that installs without systemd by default; Antix’s nosystemd repo, which allows to install components of the Debian stack without the enforced systemd dependency; and later libam-elogind-compat which aided shimming some of systemd’s requirements under elogind.

Nowadays at least, the only hard part of not using systemd in Debian is 1.- switching (from or to) seems to require rescue mode and 2.- you lose some of the container management goodies (for eg.: Podman services).