The best schtick at FBG is the one where people whine about parents nowadays being overprotective and brag about how their parents used to let them just disappear for 10 hours while they rode their bike across the interstate at rush hour, and then when some parent turns their head for long enough for one of the multiple kids she's watching to scoot under a fence they're the first ones to scream "take the kid away!".
No one has eyes on their kid every second of every day, even at a zoo (you reasonably expect that a kid is not going to be able to just climb into a gorilla pit). It just so happens that when you spend 5 seconds looking away it doesn't typically happen to coincide with the same 5 seconds that your kid found a way into a dangerous situation around safety measures. I'm sure they're not the first parents to look away from their kid for a few seconds near that gorilla pit and I'm sure he's not the first kid to try and slide under the fence, it's just the first time they happened simultaneously. And there were probably other variables too, like maybe the kid was super skinny to fit through there whereas most kids can't fit. Like Tim said, a series of unfortunate things that all had to happen simultaneously.
Not to say that the parents are absolved of blame but it was a mistake that every parent makes, they're just usually not so unlucky as to have it happen at exactly the worst possible time.
Of course we don't know if the parent was looking away for 5 seconds or 5 minutes but I have to think that if the kid was a mastermind cutting away the fence with a buzz saw for 5 minutes that someone would have noticed, even if not the parents.