1. Google builds a new process architecture into Chrome as a product differentiator. (It was a major part of Chrome's initial marketing)
2. WebKit 2 is built (mostly by Apple?) to bake the same type of architecture straight into the core framework -- anyone using WebKit can use it and get the same security/stability benefits.[1]
3. Google says that the pain in maintaining their separate, non standard, process architecture is too much of a burden to continue to contribute into WebKit proper, so they must fork.
Why can't Chrome implement WebKit 2? Are there major advantages to Chrome's process model that are not present in WebKit 2? Is there a reason why WebKit 2 cannot be patched to provide those advantages?
This seems like a failure of open source.
[1]: see the first paragraph on http://trac.webkit.org/wiki/WebKit2