Best i can tell, init as more than what started processes at boot, and stopped them at shutdown/reboot, only really became "important" as people used laptops as oversized mobile phones.

And as the concept of containers and *aaS/cloud took hold, some of the same things that benefited said laptops was found to benefit containers.

The reason systemd happened over any of the others though was that perhaps the main dev had a penchant for NIH solutions, and worked for the 800 pound gorilla of the Linux ecosystem...