Technical stuff aside, a major reason why Flatpak does not get the hate Snap does is because users don't resent it.
There are 2 things people hate: the way things are, and change.
Ubuntu put Snaps on users desktops without asking if they wanted them. They just did it, then closed off ways to opt-out/avoid them (Cf. Firefox).
Forced change fosters resentment.
No distro made Flatpaks default/required/opt-out. Users had time to come to the tech and adopt it on their own terms/needs.
Just IMO.
One can also ~easily do something similar to what flatpak would do by invoking bbwrap in a similar fashion. That allows people who want only some of facets of flatpak to get them, without resenting that they have to use the others.
(I don't know whether the same's the case with snap, though.)
