Hours are normally based on the season and attendance projections. It's not summer yet, and is actually an off season time now. Yes, the park is beautiful at night, but it's a significant economic loss to stay open when there are not enough people staying late. Most all of the parks in the area follow similar schedules.
Normally, an event will not close the park early unless it is very significant. In such a case, the event is usually as large or larger than the attendance of the park was for the day.