Flatpak is winning on Linux because when every app has its own dependency chain of packages it becomes totally impossible to run older versions of an app or to run multiple versions side-by-side. Proper sandboxing is way too hard when files get strewn all over your system. rpm/deb packages only work for utilities and core libraries where you're happy to run whatever the latest version is. For everything else you need some combination of containerization/virtualization for security and robustness.