The boring answer is probably "because defaults matter". But you are of course right that it would be less work for all people involved if these were instead D

The boring answer is probably "because defaults matter". But you are of course right that it would be less work for all people involved if these were instead Debian Derivatives or Fedora Spins where the useful infrastructure, developer processes etc. are already in place. I guess it all comes down to branding, and the fact that starting over is easier right at the beginning.