My guess would be that it's because jails are exclusive to FreeBSD, and not that many people (compared to Linux, that is) run FreeBSD. Jails were also devised as a tool for the sysadmins toolbox, whereas docker is a tool for developers toolbox - and each has its own strengths and weaknesses.
Finally, jails do lack a bit of the functionality that lets docker do some things - but that isn't something that can't exist, an in fact there are certain signs that such instrumentation might be in the process of being written: https://twitter.com/FiLiS/status/894651614002393088.
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My guess would be that it's because jails are exclusive to FreeBSD, and not that many people (compared to Linux, that is) run FreeBSD. Jails were also devised as a tool for the sysadmins toolbox, whereas docker is a tool for developers toolbox - and each has its own strengths and weaknesses.
Finally, jails do lack a bit of the functionality that lets docker do some things - but that isn't something that can't exist, an in fact there are certain signs that such instrumentation might be in the process of being written: https://twitter.com/FiLiS/status/894651614002393088.