Because it requires someone to maintain it , it is another ISO to build and test, potentially new help center articles to write, adds to stack complexity and creates a new, unique support burden. We have enough to do already without spreading ourselves thinner adding every DE under the sun.
If between our Solus editions of Gnome, Budgie, Mate and Plasma (There is also fluxbox, openbox, bspwm and i3 in the repository) a user can not find something they like so they switch to another distro that fits them better because it provides Deepin, Pantheon or Cutefish etc, that is perfectly fine.
