This is clearly for reasons of security.
I don't think Apple is terribly interested in market share for Safari. What they are interested is preserving their competitive advantage in privacy.
The security/privacy argument has been debunked many times.
How do you explain that all other OSes, including Apple's own macOS, manage to allow other browser engines?
Do you think the iOS team is that incompetent?
> I don't think Apple is terribly interested in market share for Safari
Google pays Apple $20B a year because of the market share Safari has on iOS.
I'd call that "interest"
That's 10% of their turnover (and likely mostly pure profit, as they seem to spend a fraction of that on Safari)