I think the easy answer to that is “because it is not as trivial as forking a small app that could run off of a git repo”, it’s a whole operating system involving a lot of infrastructure and a huge community around it.
This It's just an excuse. If the authors of the open letter are active developers and reflect what the majority of the community thinks then they already have the infrastructure (or big part of it, else how the fuck they work ?) and the community behing them. Man, it would not be the first time a distro to be forked.