Every time the fact of Apple not permitting other browsers is brought up, someone always says "Yes they can" and points out Chrome, Opera, etc. But they can't. Apple won't allow any code interpretation in apps. That means no JavaScript at all. Which means a browser that's pointless on today's internet.
Opera Mini isn't a true browser and offloads processing to external servers because they can't interpret JavaScript locally (thanks to Apple's anti-competitive app store rules). The off-device rendering makes for a less-than-desirable user experience.
Chrome and every other "browser" in the iOS app store is just a custom UI on top of Mobile Safari. And it's not even full-speed instance of it since it can't use the Nitro JavaScript engine. It's hobbled so it's slower than proper Mobile Safari. Chrome does add one other custom bit in that it inserts its own network stack underneath it. But it's still Mobile Safari within Chrome. It's not the Chrome/Blink engine and it likely never will be. Not unless those increasingly high walls start coming down.
It's not like iOS users can install an alternate browser, either.