>why does device enumeration belong as part of init (udev)?
It isn't part of PID 1 but hardware initialization is a dependency for services being started. The service manager ends up needing to know about hardware state for dependency purposes and therefore shares a lot of infrastructure with udev which is why they are distributed together now.
> why does logging belong as part of init (the journal)?
This is also not part of PID 1 . Whether system logging should be tackled as part of the systemd project is debatable. It is still an optional component though and might solve some real problems.