Because as the old name indicated, xdg-app is/was a Freedesktop project (though much of the docs are at Gnome, making one ponder porous project boundaries). And Freedesktop is all about defining that one canonical (heh) distro (making "free" something of a misnomer at best).

Observe flatpak becoming part of Fedora shortly, and then Poettering style "nudging" implemented to get Debian and the rest to adopt it.