The only reason, AFAIK, is because it's of strategic advantage to the systemd project, and by extension, Red Hat. (If someone has evidence to the contrary, I'd love to hear it.)
I've used systemd since mid-2013, and since then I've acquired a fair few reasons to dislike it, but it's the management of the project that bothers me more than any technical aspect. The systemd modules all seem to depend on the process manager and journal. The process manager requires that systemd also acts as init,* and user instances require a root instance. None of these dependencies need to exist - even the journalling library could be replaced by a shim that just forwards everything to stderr. Traditionally they would have been separate projects and such dependencies wouldn't exist.
* Systemd is a much better process manager than SysVinit, but there was never any reason to prevent the user from choosing another init.