Flatpak is a solution to package management as Docker is to servers. What's the predominant reason people use Docker? Because it's impossible to get a Python/NodeJS monstrosity working properly without it. On rolling release distributions, you don't ever need Flatpak. Its predominance is found in stable release distributions like Ubuntu and Fedora. Because they stall packages and their dependencies, it'll always be an issue to install new software. If anything, I prefer AppImage a lot more.