Here we go again... Some historical reasons, and some technical. Here are a few (strong points) that comes to my mind:

Ubuntu has a track record to do their own thing and splitting community or the entire Linux eco system, such as with Unity, Mir and Snaps. Unity was badly implemented desktop environment in the beginning and lot of people didn't like it. Although I think it was a great DE over time, it was just another fuel in the fire. More serious problem was Mir. Mir was an alternative to Wayland, because Canonical was not happy with Wayland and they didn't want to implement what Ubuntu tried to do on phones. But that meant the programs and protocols to support was now X11, Wayland and Mir. And related to it, the focus of mobile user interface on desktop (Mir+Unity) was something lot of desktop fans didn't like at that time. Canonical gave up on Mir and Unity (and smartphones entirely).

Snaps were very slow at the beginning, some people does not like that it fills the device loops, and not many apps were available as snaps. Snap is a similar but alternative technology to Flatpak. Again, because Canonical was not happy with Flatpak and Snap truly has some advantages over it. But it means splitting the eco system once again. But what made it really upsetting for many is, that not only Snap is pushed by Canonical a lot, but also when installing a native package, the package manager would silently install the snap version instead. That is sneaky. And not only that. The Snap repository from Canonical is proprietary. And if you want support Snap, you have to use that repo or use your own repo. Unlike Flatpak, you cannot have multiple repositories. That means in Snap you can only use Canonicals proprietary Snap repository, because otherwise you would not have all the apps in it.

There was some Amazon related datas send to Amazon with an app, every time you searched in the search bar of Unity. Even though this is gone for long time, it still is something people (me included) remember. Some say it was spyware... which is kind of was, but is up to debate.

Also some do not like that Canonical is a corporation. I personally don't have a problem with that (and used it for 13 years exclusively), but its something to mention what problems have with. Also Ubuntu is used in Windows too, so people have conspiracies too or do not like their cooperation. I'm fine with that and actually like that Linux gets more exposure this way. But again, some people don't like it.