Closing both of your parks at 5pm for a hard ticket event when you still have plenty of the general public that wants to spend time in and spend money in the parks. Plus, if you are staying on-site and don't have a horror nights ticket, that might become a bit of an issue with guests.
If Universal can close USF and make a bunch of money on a hard ticket event and ALSO keep IOA open for everyone else who just wants to go to the park (and also spend money), then they are going to ride that train for as long as they possibly can.