Google's not pushing it hard on iOS, and it can't offer enhanced features or tailor (ahem) its sites to run better on the Chrome engine on iOS. That makes it a

Google's not pushing it hard on iOS, and it can't offer enhanced features or tailor (ahem) its sites to run better on the Chrome engine on iOS. That makes it a harder sell, and the "Google sites all works better on Chrome (because we made sure they do...) download it here!" ads they had success with on desktop would simply be lies, if they tried that. They also can't get other apps to embed their engine so they get those sweet, sweet analytics from those, too. FB would likely do the same (probably with a fork of Chrome, for obvious reasons).