Because the software he writes starts out good, and solves problems. systemd is a really nice initd replacement. Pulseaudio really improved audio on Linux. Distros adopted them because they were good.

The problem is feature creep, exactly like the OP post. For some reason, Poettering's projects can't contain themselves to a problem space. Converting init systems is a lot of work, and even if Debian had recognized the feature-creep of systemd as undesirable, there was no way they were going through all of the pain and suffering of another migration. Plus, there isn't yet a clear successor to systemd. My money is on dinit; s6 is simply too complex, and has too many commands to remember. But the point is, systemd was an excellent initd replacement, and there was a lot of adoption when that's all it was. And as it grew, distros were already committed and stuck with it (although, journald was there from the beginning, and that should have sounded warning bells).