Performance, probably. To sandbox it I imagine it would have to run in a separate process, and then you'd have to pass messages back and forth, which might be slow. But who knows.
It may be one of these things they were planning to do later, and then someone pointed out it would possibly be detrimental to what they want the iOS ecosystem to be, so they scrapped plans.
Message passing between a UIWebView is already one of the big complaints, requiring slow and klunky workarounds. To receive a message you detect URL anchor (#foo) changes in objective C. Sending is a bit better (inject a js string).
I wonder if its possible to sandbox a thread as opposed to a separate process. I doubt that Apple consciously neglects HTML5 app SDK functions. iOS has been the best platform for HTML5 for a couple of years already, supporting features (CSS 3d for one) that Android didn't.
It may be one of these things they were planning to do later, and then someone pointed out it would possibly be detrimental to what they want the iOS ecosystem to be, so they scrapped plans.