As i understand it, systemd provides some functionality that some people wanted, but not everyone was happy with how this functionality was implemented and/or provided (which included people who didn't need tht functionality in the first place) and others were annoyed by how it was pushed/promoted and became a dependency on some popular projects (e.g. Gnome) without a technical necessity for being so, feeling like it was forced on people who didn't wanted it but also making it harder to use other init systems (this is especially important since, as i already wrote, the provided functionality is desired by some people and could be provided in a better way by another init system).

(note that with functionality here i do not refer to specific APIs or interfaces, but higher level functionality like "i want to be able to do XXX when YYY happens")