I imagine it's mostly build system scripting. FreeBSD builds using makefiles, and the BSD make is not the same as GNU make, especially for the sometimes-compli

I imagine it's mostly build system scripting. FreeBSD builds using makefiles, and the BSD make is not the same as GNU make, especially for the sometimes-complicated scripting they have to do inside of them. Adding cross-compilation to a complicated set of makefiles isn't impossible but can be some work. Beyond that there are other issues... eg. The ELF binaries generated need to declare themselves as FreeBSD binaries. A C compiler set up to produce Linux binaries won't do that. Maybe also they depend on a specific compiler (FreeBSD is using clang lately). When you build FreeBSD you also build a compiler so maybe this part isn't such a big deal.