It is because systemd confuses what should be split concepts into one big overarching thing.
'org.freedesktop.systemd1' (the actual systemd init D-Bus interface) somehow is still implied that it is vital to the entire systemd-something ecosystem while it's not really.
A example being systemd-hostnamed and hostnamectl complaining that 'System has not been booted with systemd as init system (PID 1). Can't operate.' [1] while in fact they use 'org.freedesktop.hostname1.${something}' [2] and that is supposed to not be bolted with the entire rest of systemd besides using sd_bus.
[1] https://github.com/systemd/systemd/blob/93258c7d72fae23c9f81... [2] https://github.com/systemd/systemd/blob/93258c7d72fae23c9f81...