Except both RPM and DEB are fully open-source. Flatpak is open-source, Snap is partly proprietary.

If it is trivial, why is Canonical keeping it proprietary?

There is a problem with pushing tech if that tech is proprietary — such as with Snaps.

Unity I don't think was ever that controversial, except that Ubuntu was sending all desktop search queries to Amazon at one point, which was, of course, terrible privacy-wise. The reason why Unity died is because Ubuntu decided it's not worth the money to maintain it.