The software philosophy of the maintainers and their choice of packages and design.
A simple but important difference: the package-manager: apt, dnf or pacman (there is more but let’s bring it down to these three).
Another one is security: apparmor or selinux
The last one are preferred and preinstalled Desktop-Environments.
And if you want to keep it simple, just be based on another distro and let them do the hard work.
Everyone can start their own distro. Manage some packages together, choose for example: Based on Arch, pacman, selinux and hyprland-wm and name it hypearch. Et voila!
Why not all of them?