Because the distros that the major distros are built on adopted it.... mainly.
Debian switched to systemd which in turn pushed distros like Ubuntu on to systemd, which then resulted in Linux Mint (based on Ubuntu) shifted on to it as well. Red Hat (and thus CentoOS and Fedora) are responsible for systemd in the first place, with Lennart Poettering, Kay Seivers and Harald Hoyer all being Red Hat engineers and the authors of systemd.
Have a quick check on Distrowatch to see how many of the top 20 distributions are based on these two base distros, it pretty much covers the large majority of installs out in the World.
There is also the issue of desktop environment dependencies. As the article mentioned Gnome, KDE and Cinnamon somewhere along the line depend on a library that is systemd dependent, with Gnome going full steam ahead with systemd reliance.
Arch Linux is systemd based but also offers a way to remove it and is also available systemd-free via Manjaro-OpenRC, otherwise I personally am just ever hopeful a number of traditionally Debian-based distros will move over to Devuan.
Biting the hand that feeds IT
