> over Debian/Ubuntu

And over Fedora/RHEL. If I had to guess, it could be that new entrants find it easier to submit changes to Arch Linux packages [1]. ChromeOS also steered away from Debian-based distributions, choosing a Gentoo base.

[1] https://gitlab.archlinux.org/archlinux/packaging/packages

They're not though. They're supporting debian and bazzite which is fedora based and have worked with fedora extensively. See https://frame.work/de/en/blog/framework-sponsorships

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