Can't speak for anyone else, but while I think jails are "better" for a variety of reasons it seems clear there is far more market momentum behind Docker. More particularly, its toolchain. Given the choice between building myself (or relying upon a small, but talented, pool of devs) and drafting off tens of millions in VC cash and a much larger community I think some would prefer the ability to have a *BSD CI chain that is just a config setting in a Docker setup.

Containerd is just one part of what Docker does. There is also image management (signing via Notary), distribution (pull/push to the Hub), orchestration (Swarm and friends), linked services (Compose), etc. You could reuse all the rest of it while running containers in FreeBSD jails as the method of containment.