Well... I'd argue both of them failed. Snap is well-made, but mired in Canonical politics to the point that it's unusable. Flatpak has good intentions, but is generally poorly maintained and driven by ideologues rather than project managers and top-down leadership. That's without mentioning the technical implementations of either project, both of which fail to eliminate redundancy in their runtimes and feel completely foreign to the native system.
Nix shouldn't have to come up so frequently in these discussions, but it's a much better package manager than Flatpak or Snap (isolation technologies aside). If Flatpak also worked with software maintainers instead of embracing mutual distrust, it could be faster and lighter on the system. Instead of tracking our dependencies properly, we've resigned ourselves to just targeting "Another Competing Standard" and hoping it won't also become irrelevant in 5 years when rpm-ostree-2 drops. It doesn't strike me as a resilient or reliable technology in the long-term, especially on it's own.