Linux

main selling points are isolation and having the latest version directly from developers without having to wait for your distro to package/update it.

both are debatable since they are not as good as promoted (isolation doesn't always work correctly and it's a mess to configure it once you use anything different than the more mainstream distros) or goes against the historical preference (using bundled everything instead of cooperating with your distro packages and trusting every individual over trusting your distro as a whole) but having the latest version on any distro without having to wait is a popular need so they gained traction quite fast. this might make little sense for rolling release distros (arch, nix) but it's helpful if you have a stable base (years old debian) but need the latest feature on an specific application or have to use very specific libraries that are not packaged on the main distro and would require complex upgrades

You answered your own question. Arch and Nix solve the same problem Flatpak solves, but by using better dependency management. Flatpak’s main proposition is built-in sandboxing and convenience, but if you’re on an “expert” oriented distro like Arch (btw), you probably don’t care as much about those “freebies.”