They must feel that it's a long enough day for most guests to do most things, and when you close earlier, it drives the guests into Citywalk where the restaurants and clubs are to make more money. In addition, when they close it is more or less a soft close unless there is an event. If they close the lines off at the closing time, guests may still be in line for the most popular attractions for quite a while. And the shops stay open until the last guest leaves (just like at the Disney and SeaWorld parks). I'm sure if attendance keeps increasing and the demand is there the hours will increase as well.
I just have to imagine that getting the park cleared for HHN will be a cluster f--- this year if there are 2-3 hour lines for Potter attractions. They'll have to cut the lines in the middle of the afternoon in order to clear the park for the opening of HHN.