My guess would be that it's because Flatpaks are easy. You have a handy GUI tool often pre installed that includes search and one-click install.
If you want something lower level, Arch users have the AUR, and others may actually do that horrifying curl https://... | sh
pattern.
Nix pancakes on the other hand.... I have no idea how to use them and generally assume it's the thing NixOS uses. Since I don't use NixOS, I've never given them a second thought.