I can't remeber who told me but I heard that it opens later because in the early morning there's a lot of mist over the Jungle Cruise, you can actually see it from the Polynesian beach, there's just a cloud hovering over that area. It can be so thick that guests wouldn't be able to really see anything, so they open it later as the mist is pretty well settled by then. I'm pretty sure this is the reason for the later opening.