Are they so different that it’s justified to have so many different distributions? So far I guess that different package manager are the reason that divides the linux community. One may be on KDE and one on GNOME but they can use each other’s packages but usually you are bound to one manager
And different goals. A large part of why apt is the way it is comes down to the way the Debian project is structured - a project relying on a large number of volunteers vs something like Red Hat where most of the changes come from employees, so has different rules and standards for how packages should be constructed