The main reason is that Chromium uses a different multi-process architecture from other WebKit-based browsers. So, over the years, supporting multiple architectures has led to increasing complexity for both the WebKit and Chromium communities, slowing down the collective pace of innovation.
The WebKit maintainers wouldn’t let us attack Apple directly, by changing WebKit in ways that would make it perform badly on OS X and iOS.
Because they share a rendering engine, developer effort to ensure Chrome compatibility currently benefits Apple platforms for free. To prevent this, we must make Chrome and WebKit behave differently.
This is a selfish, greedy move by Google which does not bode well for browser users or web developers.