Say your school has 500 kids. 1/10th of their parents work nights. They either have to miss the performance, or have to arrange to miss work for it. That's 50 parents (or 100 if it's both parents). Then... if they allow parents who are working nights to come (so they don't have to miss work) then you also have to allow all the parents who due to other commitments it would be more convienent to show up in the daytime. So everyone with a newborn, or kids in sports, or, or, or, or. Pretty soon you have half the parents in the daytime and half in the evening. Some families have 2 parents, but many have 3 or 4 (Steps) plus grandparents, siblings, etc. That's what school administrators have to look at when 1 parent wants to come to the student-only showing. And of course... all the fallout. A student-only showing just means shuttling kids back and forth from class. Having it open to parents means taking an addition 1-2 hours out of the school day (parking, arriving, asking questions, just wanting to borrow the teacher 'for a moment', photos with their children, some kids being given flowers / some not, parents wandering halls/ talking with each other, the slow trickle of leaving, the number who stop off in the office since they're already there, etc. so forth and so on). It's a logistical nightmare. Which is why they block out several hours *after* school hours in order to deal with it. Even just ONE classroom of 30 means an average of 75 - 100 adults on campus. Aside from the logistics... During school hours, if ANYTHING were to happen to any of the other students, then the school is culpable. After school hours it's understood that the parents are responsible for minding their own children. It's still on school grounds but it isn't 1 teacher responsible for 30 kids and 75 adults. It's 75 adults responsible for 30 kids.
WILL some parents show up for the student-only showing? Absolutely. But as long as that parent is breaking the rules in order to do so... it doesn't create the enormous problems of half the parents on campus.