My 2 cents: init script wasn't a enough pain in the ass for people in charge to search for alternatives but still a pain, and systemd was introduced to them by Red Hat manpower. It seems good enough so they went with it.

Actually, init script wasn't anyone's enough pain in the ass in Linux world save for djb. daemontools came out in 1997 and no one seemed to care. Systemd replaced those init scripts IMO only because they wrote all the replacement service files as well with the manpower at Red Hat's disposal. It's hard to match that from open source world.