Adam Barth of Google basically says that because Chrome uses a different multi-process architecture compared to other WebKit-based web-browsers and multiple architectures, the situation has become rather complicated for Google developers.
Blink is to be an open-source rendering engine that will in time deviate from WebKit. Key goals with Blink include performance and simplicity. Google also believes this fork and more rendering engines will spur further innovations than most everyone relying upon WebKit. Google's routine fighting with Apple is also likely another reason for parting ways with upstream WebKit.
Initially this fork has internal architecture improvements and a simplified code-base, which already lightens the engine code-base by four and a half million lines of code.
More details on Google's Blink can be found from this Chromium blog post and the Blink project page.