Audits aren't supposed to be an ultimate guarantee of security, but provide a minimum, independently judged hurdle that has to be passed to get on the platform.
If there's a better, independent way to judge what browsers are "secure enough" to be on the platform (ie. not just "Apple says no"), I'd love to hear about it.
Minimize attack surface, minimize choice.
As I said in my comment: I believe Safari and the underlying WebKit to be the most complex and most insecure part of iOS by multiple orders of magnitude.
Not adding more of equally complex and demanding pieces does provide a significant reduction of attack surface