My guess is that Arch is easier to build on top of because they have a stronger culture of leaving packages as unmodified as possible relative to their upstream sources, whereas Debian maintainers seem to have the opposite culture. A Debian system has a lot of Debian-isms in it overall, whereas the Arch-isms tend to be more like generic sensible defaults rather than OS idosyncrasies.
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
> Arch Linux is an independently developed, x86-64 general-purpose GNU/Linux distribution that strives to provide the latest stable versions of most software by following a rolling release model.
- https://wiki.archlinux.org/title/Arch_Linux
> This page complements the Installation guide with instructions specific to Apple Macs. The Arch installation image supports Apple Macs with Intel processors, but neither PowerPC nor Apple Silicon processors.
(emphasis mine)
- https://wiki.archlinux.org/title/Mac
(FWIW, I understand that there is benefit to good coverage of a narrower scope, but I do wish Arch would fold https://archlinuxarm.org/ into the main project and be officially multi-arch, but that is not the world we live in.)
Arch (or even Gentoo) is great if you want to do more detailed customizations of various things.