*Ubuntu Proliferations?

[Fritz]

Does anyone have any idea why Canonical makes separate
distributions for Ubuntu, Kubuntu, Xubuntu, etc.?

For example, I seem to recall other distributions that let you pick one or
both of Gnome and KDE at install time.  Later, depending
on whether your ".xinitrc" is set to "gnome-session" or
"startkde", one of the two desktops is activated when X starts up.

Is there some real (technical) reason these are separate
distributions?

Fritz