SystemD is a Linux cancer that continues to grow uncheked. If this keeps up, a base Linux system will be nothing more than the Kernel, Systemd, and Bash.
I could see distros like arch, that want to test new stuff and are known to work on bleeding edge software would make sense.
That's a bit disingenuous, Fedora 15 (released May 2011) was the first distribution to enable systemd by default, so there were at least 2 years of "rollout and experience" when Debian 8 (April 2015) and CentOS 7 (July 2014) switched. And at least in the case of Debian, it was already available as an option since 2012.
And yes, while available in debian, very few people used it for the reasons I'm talking about: it hasn't been heavily tested.
Debian ramming systemd into stable like they did on the other hand...