Why did GNOME and basically all the large distros all jump with both feet in on using systemd? Because it was better. It was simply significantly better than all the alternatives. For the vast majority it was a no-brainer upgrade. The holdouts were the one's who had simple needs and were already happy with what they had. The rest of the world jumped on systemd. Because it was better.
GNOME and systemd teams were in many ways joined at the hip, and GNOME unilaterally decided that from 3.6 to 3.8 they would switch certain APIs from one already deployed widely (polkit and related) to one that was documented like north korea is democratic (logind) which also didn't work in isolation from systemd.
Trying to run GNOME 3.8 without logind caused significant problems and instabilities, trying to implement the same APIs turned out a futile endeavour though one OpenBSD guy got sufficiently motivated and kept patching GNOME for OpenBSD for years - though too late for the forced switch.
The large distros jumping "both feet" on systemd were essentially Fedora/Redhat (where it originated and who was employing major maintainers), and IIRC SuSE. Arch was still seen as something of niche and - crucially - was very neophyte about adopting systemd related ideas for significant amount of time with little regard for stability.
The holdouts were not just those who were happy with debian/redhat simplistic run-parts script. They were also those interested in solving the problems in different way. Hell, systemd was pretty late to the party, the major difference was that it had funding behind it
