I guess because otherwise they would have to have a different code base for Windows, Linux and OSX. They are probably relying on Chromium to solve the multiplatform aspect of this.
And probably because it's more "stable" in the sense that the bundled chromium won't change independent of the rest of the software.
Plus storage is cheap and plentiful, and while webkit is pretty close to chromium, it's not identical and then they would need to support 2 separate "platforms", even if only for that 2% of differences.
Plus storage is cheap and plentiful, and while webkit is pretty close to chromium, it's not identical and then they would need to support 2 separate "platforms", even if only for that 2% of differences.