Part of it is that Ubuntu/Canonical so aggressively pushed Snaps which became a huge culture war. So you have people who hate the idea of those style of packages because they hate Snap AND people who hate flatpak because they are Team Ubuntu for some reason.
And the other aspect is that it is incredibly space inefficient (by the very nature of bundling in dependencies) and is prone to "weirdness" when it comes to file system permissions and the like. And many software projects kind of went all in on them because it provides a single(-ish) target to build for rather than having a debian and an arch and a redhad and a...