Linux distributors frustrated by Google's new Chromium web browser restrictions

Packaging: I suspect Google prefers to centrally distribute Chrome binaries, so they can swap out versions at whim, potentially even split-testing at the distributed packages. Most distributions push the "get a signed RPM/DEB/etc from our repository" model, whose trust would be shredded by jumping out to download from Google, so they supply what they can package themselves: Chromium.

Builds: I recall for years the sales pitch of distributions like Arch and Gentoo was "everything ends up compiled with the exact optimal flags for your machine, so it runs faster than the generic x86-64 build that has to run on a first-generation circa 2005 Athlon 64." That's only supportable with the Chromium source package.