Yes the core is BSD derived unix, but OSX is also a massive layer of libs on top that is proprietary to OSX.
So unless the developer takes outmost care to not touch anything that makes OSX, well, OSX, its damn hard to transfer anything over to a _nix from OSX.
In other words, writing portable software is harder than writing non-portable software. Fair, but I don't see what it has to do with the matter at hand.
Yes the core is BSD derived unix, but OSX is also a massive layer of libs on top that is proprietary to OSX.
So unless the developer takes outmost care to not touch anything that makes OSX, well, OSX, its damn hard to transfer anything over to a _nix from OSX.