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
Convenience/ease of use; There's a little caveat that only "true linux users" are able to notice, tho –
… compiling is, and always will be, the best option.
And that's why Portage is the best package manager
Gentoo has alternatives to Portage too.