Google has given a reason that All apps on iOS have to run in the sandbox environment except for special apps by apple that get to tap into some private APIs.
So Chrome on iOS uses whatever the UIWebView provides plus it might be doing its UI stuff and some external caching to add something extra to it.
But Why does Google need access to Nitro engine that Safari on iOS has access to.
Can't Chrome port the whole webkit engine and V8 for iOS ?
Xamarin 2.0 manages to port mono (.net runtime) on iOS along with every app.