Jails are engineering based.
It may make no difference to you, but it is the same difference between religion and science.
You don't hide problem behind nice easy tools easily deployable, you do it the hard way.
And as BSD says: jails are strong cages, but they are made of glasses, and when they break, it breaks badly.
Engineers don't believe in reassuring technology, they control them and do not fear the ugly details, they are aware of the domain of validity, accept imperfections and do not hide them under stickers, propaganda and success stories. They do risk control, they right manuals and point out weaknesses.
Jails user at least know this solution is a brass bullet and can fail you.
And sometimes good users worth more than good technologies.
A good soldier is valuable more than good weapons.
That's why early adoptant of linux are going to BSD. To get the control back and go back to basic engineering approaches that are stable.
I learnt in this process that you can learn from the mistakes that made linux fail the community : evangelization, consensus, philosophical approach, over-promising and being nice.
Jails are in the system.
On the other hand, I associate Docker more with the machinery it builds on top of containers. It's got all the snapshotting and build/distribution infrastructure. I think popular wisdom is also only to run a single process in a Docker container, whereas most people I've seen (myself included) treat jails a lot like a virtual machine.
Jails are really popular amongst FreeBSD users and I use them a lot myself, but FreeBSD is nowhere near as popular as Linux, so that's one thing that held it back. Also, I think a big part of what has made Docker popular is all the deployment and distribution stuff on top of bare containers. The whole Docker philosophy of mostly immutable containers and microservices was what people wanted and FreeBSD jails aren't really intended for that.