Because it doesn't "just work" in the actual sense: namely, installing is an involved process. That's great for people who are into Linux, but that's not your audience if you're talking about growth.

Manjaro may have some hope: it's received a lot of criticism for the odd way the maintainers have set up some aspects of the distro, especially from Arch users. I've seen HN threads full of comments saying things like "why on earth would anyone use Manjaro when you can use Arch?". Those commenters miss the selling point of Manjaro: it's a rolling release for Ubuntu's target audience, and that target audience want one important thing Arch does not offer:

- an officially supported, opinionated installer of a fully configured Desktop environment