Apple blocks all non-Safari browsers from the store. Opera Mobile doesn't exist in the App store, only Opera Mini which uses a remote server for rendering. Other browsers in the store use iOS webkit and are effectively shells around the Safari rendering engine.
Chrome started as "effectively a shell" around webkit. As far as most end users are concerned, the difference between browsers is the UI, not the rendering engine. I use iCab on iOS for its UI and amazing feature set (including Adblock and extensions), and apprececiate the choice. In fact, it syncs with Firefox bookmarks. Atomic is also nice.
It would be very interesting to do a Firefox-like UI for iOS and see what the end user uptake would be.
