I'm sure for the vast majority of Chrome OS users, the technology powering their device is of no significance compared to a reliable and consistent UI.
plus who cares? Chrome OS could just be a stripped-down android, and the world would be fine. Actually I though they were already mostly the same (I have a chromebook, but no android device, I don't know what's the difference, appart from the fact that my chromebook gets regular updates)
There are flavors of android including vendor modified or changed specifications. They can be subtle, like a pay as you go phone carrier, or major like what Amazon has done with Fire. Each of these changes introduces security issues outside of Google's control.
With a Chromebook you get the ChromeOS from Google regardless of the vendor who made the hardware.