Well, distros haven't really put any effort into making it viable as far as I know. They really should! Why isn't there a standard Linux package format that all distros support? Flatpak is fine for user GUI apps but I don't think it would be feasible to e.g. distribute Rust via a Flatpak.

(And when I say fine, I haven't actually used it successfully yet.)

I think distros don't want this though. They all want everyone to use their format, and spend time uploading software into their repo. Which just means that people don't.