This is a joint decision from all Ubuntu flavors. It frustrates me so much that every Ubuntu-wide decision gets framed as "Canonical does x". It ignores the thousands of volunteers who pour their heart and soul into this project. The fact is that we, as a project full of volunteers, have decided that Ubuntu is built on debs and snaps.

One of the core features of Ubuntu is that you get 5 years of support for your software after release. We go through great lengths to ensure that whatever you install the first day after release will continue working for 5 years. We simply can't ensure that with a third-party repositories like FlatHub. This is why the Ubuntu Technical Board decided a long time ago to not ship third party repositories by default. It doesn't matter if these are ppa's or Flatpak remotes. Any flavor that was previously shipping Flatpak, shipped it without any remote, even before the decision just posted.

As Aaron said it so well on the discourse: "Most of the Ubuntu flavors provide free technical support via forums and IRC channels. Most of our users are using software from the Ubuntu repos or Snap Store and we are equipped to help them. We know what to expect from the software our users run and can give targeted and efficient advice on how to resolve issues. Some of us can even kick things into shape in the archives if there’s a legitimate problem with our packages, or we know who to talk to.

With Flatpaks, the situation is much more dismal from a technical support perspective. We have little-to-no clue what quirks the software vendor(s) will have introduced since we don’t work closely with them. We have no way to reach in and fix legitimate bugs aside from filing bug reports and hoping that they will be answered. We’re going to end up with frustrated support staff and even more frustrated users. And all because they didn’t know that if they clicked a particular button in their flavor’s app store, they would be downloading unsupported software."