I think the reasoning was that you shouldn't need systemd in docker because the point is for docker itself to containerize each service unit, and use something like docker-compose to manage multiple units. Personally I think that's an aggressive but valid stance because by allowing systemd in docker they'd get a ton of low quality docker images floating around where they launch multiple services within containers, which is against the "do one thing per container" philosophy.

Now that docker has proliferated and the standard practices are clear and well documented, there's less of a danger allowing people to do messy things in their docker containers.