If you leave parks open at night you have to provide quite a bit extra policing. Otherwise they get taken over by hookers and junkies, and normal people won't go there. That kind of policing is the first thing that gets cut when cities run out of money, at least in my area, so they close the parks at night.

And if you close it, then you either need a big impenetrable fence around the whole thing, or you still need to police it.

You do, but not as many.

In my city they close the small parks (1-10 blocks, surrounded by cast iron railings) but can't do anything about the huge park that covers several square kilometres and has important roads running through it. There are some hookers and junkies and doggers there at night, but they have to go somewhere and they don't bother the "normal people" who are, after all, tucked up in bed at that time.

All this suggests that locking parks overnight is more about every government's burning desire to "think of the children" and control everything possible than any legitimate concern for public safety.