Chrome for iOS has many features Safari does not.
- Unified search and navigation bar
- Much easier private browsing
- Voice search
- Request desktop version
- Flicking between open tabs
It's notably missing "Reader" mode. They both support syncing with the desktop version.
This is not just a UIWebView. Chrome has a decent market share on the Mac and using it for iOS makes more sense for these people. Especially for the syncing.