Systemd is great software -- it is useful, and powerful. A systemd unit file, the idea of what a unit is, when they run, how they run, what shells they use, what permissions they run under, and lots of other complexities are expressed as options in the unit file, configuration files on the system, and in other places.

If I want to run a useful piece of software like redis let's say, but I want to run it with a resource constraint to make sure that it doesn't take more than 2CPUs and 500MB of memory, it is far easier to do that with the following command line:

   docker run --rm redis --cpus 2 --memory 500mb -p 6379:6379

Than to write the equivalent systemd unit file, set up the isolated filesystems that docker would let you easily bind mount in, etc. This is like comparing systemd-nspawn to systemd -- if systemd-nspawn isn't simpler than systemd then what are we even doing.

Docker won because of it's developer ergonomics (containers weren't new), systemd won because of it's feature set, convenience and sturdiness. They're different tools with different primary use-cases.