I'm not familiar with the situation, but my guess would be that ChromeOS has created its own Wayland compositor with a bunch of private protocol extensions that Chrome on ChromeOS uses to get stuff done. But for some reason or another, those protocol extensions are not suitable for submitting as new public extensions for other compositors to implement. Or they are suitable for submission, but the official extension roadmap already has different extensions in the works (but not yet complete) that will do the same thing.

ChromeOS uses Wayland as a bridge between Android and Chrome because it's a politically neutral technology neither of them control, and thus can both agree on implementing. It's not about integrating better with other Wayland compositors or the wider Wayland/Linux ecosystem, it's about using Wayland as a piece of technology to sling buffers back and forth between two different camps who are otherwise on bad terms.

ChromeOS implements a Wayland compositor to support Android apps running as Wayland clients. Chrome being a well-behaved Wayland client is something completely different.