Because Blink is the best engine. Chromium, same as any other engine, is FOSS, therefore everyone, if want, can gutt out the Google crap it has by default, what the Vivaldi devs do very well in every update, leaving some Google APIs as options in the settings. If you desactivate eg the one of the Chrome Store, Vivaldi can't download extensions from there, because it isn't seen not longer as a Chromium. Vivaldi isn't a simple chromium fork, pasting just the own logo, there is a lot of work behind every new release.