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Isn't that obvious? With iPhone being major mobile device they put enough pressure on web developers to support WebKit which benefits Safari not just on iOS, but on macOS as well. This also not just wins better support for Safari, but give Apple some foot in making of web standards.

If they allow alternative engines on iOS they'll just slowly lose browser market share they still have which likely end up in death spiral for Safari support.

It's extremely hard to compete with Google since even Firefox on Android is still extremely rare no matter how much effort Mozilla put into it.