For the ‘How’:
Fairphone switched to using Google’s update mechanism (the one baked into the settings menu). I recall that MCC/MNC values of the last primary SIM (usually the one that has/had a data connection) are sent as part of the request to check for updates. Android also has carrier services that allow dynamic configuration of the device by the carrier, I’m not sure whether this includes the ability to set an update channel or policy. In any case, the update is only pushed to a device if the carrier has approved it, or has supplied its own version.
For the ‘Why’:
This way carriers get the possibility to quality control updates and add carrier-specific features (like VoLTE/VoWiFi, but also bloatware). So one excuse is ‘user experience’, but I personally feel there are commercial reasons that better explain why this happens.
Showing up as ‘temporarily unavailable’ for me (here). That said, the Windows instructions link points to what appears to be an unreleased page (if @rae is collecting issues, please add this one).