Adam Barth explains that Chrome already was using a different architecture than other WebKit based browsers and that the increased complexity was slowing down progress. Blink itself is based on WebKit but it will allow for a simpler code base that Barth says will spur more innovation.
Initially Barth suggests they will be able to remove 7 build systems and delete more than 4.5 million lines of code in over 7’000 files leading to a healthier codebase and increased stability. A major focus of this shift is to improve the health of the open web ecosystem and promote innovation. Google will be working closely with other browser vendors to keep the web moving forward and preserve compatibility, setting strong guidelines for new features ensuring a consistent web experience.
You can read the full announcement here on the Chromium Blog and also check out the project page to read up on Blink.