They switched because Google bundled invisible Chrome installers in other software that would not only make its browser the default, but also invisibly steal IE clicks.
Must say it's really hard to find who might have paid for a bundled installer. Filezilla I remember had a lot of those and some forum threads will mention a specific "offer" they got, but there's no listing anywhere, and it's not in the source code history because they were (so I just read) dynamically fetched from advertisement servers upon launching the installer. Searching the web for Google Chrome bundling (phrased a few different ways), you get mostly present-day results about how to install Chrome or how to bundle it as sysadmin in a Windows group policy or something. This is the one thread I found where it sounds like computer manufacturers bundled it, but if there's many more then I'm not sure I'd have found it
They did a fresh Windows install though, that wouldn't normally keep any software they previously had on there, but yes it's the best reference I could find. Feel free to find a better source yourself, I've tried
Must say I read over the bit about invisibly stealing IE clicks. That's obviously nonsense, Chrome was never malware
A move that was widely celebrated at the time.