"Objectively terrible" certainly isn't the case with systemd, but there's plenty of reasons for adopting software in general that has nothing to do with technical merit. Much of the Linux desktop daemons (particularly ones associated with GNOME and Freedesktop.org) have begun using systemd's interfaces, sometimes as hard dependencies. Thus, for the major distributions that want to tailor to the most popular use cases, the cost of adopting systemd is probably lower than patching against the ever-expanding upstream that requires it.

Lots of programmers aren't particularly good at analyzing the cost of surface convenience in proportion to future technical debt. Software is just as frequently adopted purely because it's convenient, well marketed or in a self-serving feedback cycle, because it's already popular.

It's also worth noting that ChromeOS still uses Upstart.