The cost of staffing the park and the operating costs of running rides (energy/cycles/etc.) is almost always going to be greater than the additional money visitors spend in the park in those final two hours.
They already opened the restaurant on Saturday nights from 5-9pm. If the goal is to get people to eat dinner and spend money, then they just need to open the restaurant, not the entire park.
Parks know when and where people are spending money. If it were profitable to extend operating hours, they would.