The value of Guix on macOS is that your users can use the same project definitions on macOS and Linux. I don't personally care about the "from-source bootstrap" all that much, I want tooling that I can use to express my system's configuration as code and nix does this just fine on macOS.
As far as the ideological reasons goes, last time I looked for this, someone on a mailing list asked how you'd go about porting guix to macOS and some core team member answered they had no interest in supporting a non-FOSS system.