The attendance they see during the off-season doesnt justify longer hours. Its the law of diminishing returns. X amount of people have to spend enough money to offset the cost of having longer operating hours. But the park only hosts y amount of guests. Universal knows what theyre doing. You also have to factor in the amount of employees available in the off-season, which again, have shifts that offset the costs of what guests spend in the off-season. Longer/more shifts means spending more money which Universal figures will not get re-couped.
Also, CityWalk is a huge moneymaker...getting drunk in a club is much more feasible than doing so in a theme park. Not only does Universal know this but people seem to enjoy that concept--last winter the park got noticeably more empty when it got dark, because people wanted to party, see movies, etc. and were tired of the parks.
Until Universal begins seeing significant attendance increases (which it very well may when DA opens), the park hours will remain short(er). And even after Diagon, theres no promise off-season will be busier enough to justify longer hours.