The fact that Chromium OS has been teetering on the edge of deprecation/merging with Android/Fuchsia for a decade I think has deterred people from building stuff on top of it.
It also seems to have a lot of new code every year for very few new features. It's as if they get every new intern to rewrite a bit of the innards, and then next summer another intern rewrites it again.
OTOH, it was used for multiple container-optimized distros by now:
First CoreOS, which forked into Flatcar Linux (now funded by Microsoft) and Fedora CoreOS (rewrite from Gentoo/ChromeOS base to Fedora base), and Google's Container-Optimized System (used heavily in Google Kubernetes Engine).
Unfortunately, there's no popular non-Google distro of it.