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.

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

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