I think another major problem that needs addressing (that apparently may be addressed in a few months according to this thread) is how Snaps don’t actually run the same across all distributions due to it relying on custom AppArmor patches provided to Ubuntu for some sandboxing/security features.
This can break apps or cause major security holes (as documented by a developer of the Nextcloud Snap) and occurs on most non-Ubuntu distros, which is a huge problem for a system that is meant to universal, and that Canonical advertises works on over 40 distros. One of the most frustrating parts about this to me is that it’s been this way since 2016, when the porting effort to other distros began. About 7 years later this is still a problem. That just sucks.
So I wouldn’t be shocked if this was a reason why some developers choose not to bother with it. But I also think another big reason would be the general bad publicity it garners due to the reasons you listed.