Scope creep and not-invented-here syndrome; replaces a lot of unix/gnu tooling/specifications with poorer ones, while shitting on some that made *nix great. Which is why your distro is either Systemd or not, and not-Systemd distros still need wrappers and shims while Systemd also enforces some things in apps.
Then there was only hackjob SysV scripts or Systemd, so it’s understandable that most big distros switched to it but now there’s s6, runit, Dinit.