Ecosystem. This is a limited function device, which is bad, but anything you can do on it you can also do on a full Linux distribution. Which means that anyone who can get by with such a device will have no barrier to switching if they are ever inclined to do so.
That is the difference from a Windows machine. Because if you have Windows then you start accumulating Windows apps and you become accustomed to their interfaces and your files are in those formats, etc. etc. Windows locks you into Windows. ChromeOS doesn't lock you into ChromeOS; you can switch to a full desktop Linux distribution whenever you want and everything still works.