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
Some differences can be explained. Pacman was created after the Debian package manager (I guess that because Debian is older than Arch) . It is justified because Pacman is faster than Apt. But its too much work to replace Apt by Pacman comparing to the benefits.
But in some cases I don’t know why. As instance I wonder why a distro, such as Void, created its own package manager instead of using the Alpine one. If Alpine is younger than Void, invert the sentence of course.
Void was created just for testing xbps. Without xbps there would be no Void.