Because "users who just want their apps to work" is only a subset of "everyone" (and for them, yes, Flatpak is a reasonable solution to this kind of issue).
I'm part of a different and non-overlapping subset: if something doesn't work as advertised, that isn't acceptable. If there's a distro-native package and it won't install and run, then that's a bug and should be treated as such.
If you use "everyone" when you know that there are people out there who disagree with you, you should expect to get some flak.