Waiting it to be ported to BSD systems[1][2][3].
[1] https://github.com/dotnet/coreclr/labels/os-freebsd
[1] https://github.com/dotnet/coreclr/labels/os-freebsd
The FreeBSD port was a community effort, and not done by Microsoft.
Unfortunately the small team doing the port ran out of time before life caught up with them (jobs, kids, university, etc) at around 95% completion.
After that .NET Core kept moving fast, and catching up (new libraries, llvm versions, mono versions required for bootstrapping, etc) also required FreeBSD updates, and all in all was somewhat hard.
Disclaimer: contributor to the original port.
BSD Java development is there too and doesn't get as many contributors as it could.
I have been programming FreeBSD systems for 15 years--actually longer--and it's never crossed my mind to even once think about using .NET or any Microsoft product. Makes no sense at all.