I see people hate snap packaging and removing it if their OS support it.
Is it because it's NOT fully open-source or just due to how the technology works?
Snap is not fully open source. It's slower than flatpak, it's centralized to Canonical's servers.Flatpaks so not update by default where snaps do, so if a feature breaking update is released and you haven't disabled automatic updates, you're screwed with snap. Flatpak does not need admin privileges where snaps do.