While it would be nice to have more open, I can't think of a single theme park that opens everything during early entry.
For Example:
Magic Kingdom only opens Tomorrowland and Fantasyland.
Cedar Point (Ohio): Ocean Motion, Midway Carrousel, Raptor, Millennium Force, Iron Dragon, Maverick and Planet Snoopy.
In most cases, they use that hour to get the rest of the park ready for the day. In Universal case, on top of that, prior to Cabana Bay, most of the guests who were eligible for early entry were also eligible for unlimited express. So, from a guest prospective, most guests are going to go towards the rides that don't have express (HP) or have long express lines (despicable me). Everything else, the deluxe onsite guest can pretty much ride whenever they want with a minimal wait. There's no need to wake up early for it.