This has more to do with systemd trying to be a "system layer" project for Linux, and influential projects (initially Gnome, then KDE, and now a lot of other things) going along with it.
Honestly, I see no reason a distribution couldn't support multiple service managers. Indeed, Gentoo officially support OpenRC and systemd and do just fine. However, you need to have the resources to do it and the desire. I think there are multiple distributions with one or the other, but Gentoo is likely the only with both.