From my understanding, Linux CGroups proved itself to be a viable alternative to jails, because, it provided "Linux inside Linux" in practice. Also, if my memory serves right, jails had some performance impact for the workload inside the jail.
Docker, as you know better than me, is just a wrapper over CGroups, nothing else, plus a container image format, basically.
In the end, CGroups provided what jails provided with the same and better performance, and someone leveraged that subsystem with a tool, before everyone else.