Less sarcastically: because it does too much, in ways that sometimes are impractical (networkd comes to mind). Expect better answers by more knowledgeable people.
Personally, I like it. But I’m a simple person with simple needs.
I just want to be able to have resolv.conf work. Thank goodness it still does on debian.
Unlike most Linux development, they haven’t focused on what their devs want to do (which for much of Linux is repackaging Ubuntu with uglier skins, and Ubuntu itself is a repackaging of Debian/Gnome), but rather, are solving problems that distros are actually facing.
The consequence of that is that nearly every distro goes with the systemd solution, and it’s this popularity that leads to people complaining that it’s being forced upon them.