[nerdy]

Without adjudication, it was a choice between this and init (the daemon it replaced) at a minimum.

[LukeShu]

systemd-networkd isn't a replacement for init, it's a replacement for NetworkManager/netctl/….

[lambda]

It's not even really a replacement for NetworkManager; it's designed to be used to provide basic networking within containers, for systemd-nspawn. It can be used on host systems, but it's deliberately fairly lightweight and designed for fairly simple configurations in containers, rather than as a full fledged replacement for other networking management daemons.