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.
There needs to be a name for the fallacy that the technically best solution will always win.
BSDs aren't more popular in commercial spaces because it's too difficult to find engineers with BSD experience, and no pre-existing supporting ecosystem. There needs to be a compelling reason for companies to ignore the conventional unpopularity, which means that using a BSD must solve a specific pain point that's visible and understandable enough to executives to get approval for going outside the mold. I hate to break it to you, but your boss (probably) doesn't appreciate why a POSIX API is even a thing in the first place, let alone how it applies to a business case.
Given that BSD doesn't require you to go through such hoops for a POSIX API, it's a mystery to me why the BSDs aren't more used in the enterprise/SaaS space.