GNOME, essentially. If you want to run a fully-functioning GNOME on your distro, you have to be running systemd. Or have the time and patience to maintain a large patch set to work around the systemd dependencies.
This became this way only once systemd has won.
At least during Debian systemd debate, both sysvinit and upstart could run Gnome just fine.
(also, what were the alternatives? I don't run GNOME myself, but given what GNOME wants do to, it does need a solid session management. I have seen ConsoleKit, it was worse than systemd.)
Debian's tech committee had their infamous vote about the default init system for Jessie in February 2014. Earlier, in October 2013, Josselin Mouette, a GNOME packager for Debian, wrote this:
"GNOME in jessie will need systemd as the init system to work with all its features, just like it needs the network configuration to be handled by NetworkManager."
