There is an extremely high bar for exposing a feature switch on iOS, across the system.
That bar would be exponentially higher for an experimental feature that is exposed to arbitrary Web content. In general, users can't be asked to make a decision about these things - there is too much to explain and a lot of risk.
The same applies on OS X, although there we have WebKit nightly builds that expose experimental features. We're working towards getting this for iOS too.