Because I like FreeBSD and if the kernel's compat layer got the job done, why not?
Well I mean you can run it, I was not even aware there is some effort[1] to run it.
What I was trying to say is that in my opinion this is one of inherent flaws in Docker and I think docker is really a dead end (I'm not talking about dead end of containerization, but just this kind of approach).
What people actually expect from containerization is essentially what Kubernetes is currently providing (quickly spinning new instances of processes, autoscaling, load balancing, rolling updates etc), but I already see questioning why docker is even needed at that point when really cgroups are used to restrict resources and processes already provide enough isolation.
So with these requirements you can't run it on anything but Linux.
FreeBSD actually has compatibility mode and can run Linux binaries, but why would I want that instead of running a native code?