IIUC, docker (and arguably podman) may tend to have more out-of-the-box isolation, and it probably involves less work for those who aren't constructing their own containers. I'd also argue that docker-like containers tend to be more portable, and that also contributes to their widespread adoption. That said, I'll agree systemd-nspawn containers merit more use, especially for those who are inclined to tailor their GNU/Systemd/Loonix boxes.
IIUC, docker (and arguably podman) may tend to have more out-of-the-box isolation, and it probably involves less work for those who aren't constructing their own containers. I'd also argue that docker-like containers tend to be more portable, and that also contributes to their widespread adoption. That said, I'll agree systemd-nspawn containers merit more use, especially for those who are inclined to tailor their GNU/Systemd/Loonix boxes.