Because Universal/IOA aren't full day parks in the off-season. They have x amount of employees who they only want to pay y amount of money for z amount of hours. They aren't gonna start paying people for overtime or paying for new work shifts when people can easily get their money's worth in the eight to nine hours Universal gives you to play.
Disney considers a 'good day' one in which the guests can do 8 attractions. At this time of year, you can easily bang out both parks at Universal. No extra hours needed.
Why operate 9-6 instead of 12-9? Because then the park has to pay for lights, lose money that could be made at Citywalk, and risk having people not eat lunch at the parks, which would suck because lunchtime is the park's biggest money maker.
Basically, don't count on Universal parks being open super late anytime soon. At MK, the crowds justify the hours. It sees a tonnn more people than IOA, it's the number one theme park in the world for God's sake and is comparable to Paris in terms of the sheer number of tourists it sees.