Seeing as gnome is/was nowhere near the top desktop environment, ubuntu had its own version of systemd at the time, and systemd had already garnered a huge amount of support across other distributions … no, it's not "the only reason" systemd won. systemd won on merits of practicality, because while the vocal minority hates some of the things it does (such as providing a bundled ecosystem of administrative software that all works together), it's also a big reason why it's popular across people who just want their computer to work.