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.

@omgubuntu I'd say the perceived slower performance of snaps vs flatpaks drives some of the resentment. Immediately noticeable on Firefox for example, the default Web browser of the distro pushing snaps.
@sproosemoose That's absolutely played into it. Flaws were more pronounced in the early days of Snaps, making a(n even poorer) first impression. You can't tell people "we've changed your browser to a Snap - it's better" and then give them a worse experience than what they had. They see through it. It's almost, without getting too emotive about software, borderline insulting.