I've honestly never heard of anyone bring up the GPL or GPL-circumvention in the same breath as Docker. I have no idea what hoops you're referring to. In my experience, the biggest reason most people don't use jails is because people don't use or get exposure to FreeBSD.

Nine times out of ten, the servers at your first job or university were centos or ubuntu, and your desktop tower was rhel, fedora, or again, ubuntu. If you're an enthusiast maybe you had an arch, debian, or gentoo install. (shrugs slackware predates me).

Without some degree of exposure or evangelism you'll choose what you've seen work over something you suspect might not. And that rationale applies to a lesser agree to docker vs rkt.

Nothing profound. It just has popular mindshare so it's the default. You'll have to beat the pants off something to overcome that.