It is my understanding that the back-end marketplace for snap is not open, and that snap as a packaging ecosystem is permanently tied to Canonical (company behind Ubuntu) exclusively.

No one else can build a snap repository or source (not sure what the best language would be but I'm trying not to word things ambiguously).

From Wikipedia:

Others have objected to the closed-source nature of the Snap Store. Clément Lefèbvre (Linux Mint founder and project leader[75][76]) has written that Snap is biased and has a conflict of interest. The reasons he cited include it being governed by Canonical and locked to their store, and also that Snap works better on Ubuntu than on other distributions.

Which is why people are unhappy with snap. And why I say that although I wish fedora didn't set up their own flatpak repo and provide then alongside flathub, to me its a requirement that it be possible to do that. Because then if the people leading the project start making user hostile choices, you have recourse. Same as with any free license, open source project- you can just take what was already built and the community can rally around moving efforts over to the version that isn't being user hostile.

Snap doesn't have that. If they became successful, canonical would have enormous power over the linux ecosystem and if they chose not to treat users with respect, they would already have market capture. The more successful they were to become, the more likely things depend on them. Like important packages only being published as snaps. And the more likely that things have been built around snaps specifically, the bigger of a liability it is for linux as a whole. A liability controlled by a for-profit company, with for-profit motives.

People have similar frustrations with systemd as more projects build hard dependencies on it, but at least those are still totally open projects

Sorry to the long wall of text but I hope its at least helpful 😅

Edited to add the section from Wikipedia