Maybe because not every system is Debian, and Plasma has to work on systems that either don't have /usr/share/i18n/supported or put is somewhere else?
I manage a project that encounters this sort of thing regularly; my biggest problem is terminfo entries. Not all distributions contain all of the same terminfos. It is one of the biggest source of bug reports my project gets. I've been considering just embedding all of the terminfos in my project, just so I know they'll all be there on every system it's installed.
I don't know this is Plasma's reason for including their own list, but it could easily be. It could also be because those are the locales Plasma supports, and it may not support every locale that might be in the distro system list.