Quite obviously, as many people have said here, because he is being released into a situation where those responsible for his care, knowing what they know now, will be in a position to ensure the public safety. It's not difficult to imagine what this looks like - one assumes he is in a heavily supervised care situation, but that the risk he posed was not previously identified, hence the zoo trip with two carers who felt able to be somewhat relaxed in their level of vigilance.
I sincerely doubt he has exhibited this level of dangerous behaviour before - one assumes his carers would have been more alert otherwise, and that the excursion to the zoo would perhaps not have been planned in the first place.
People can surprise you with their actions. Bur, now they know they know.
It's a horrible, tragic accident, but I don't see who will be helped by persecution and blame of the individuals involved. The gentleman who carried out the offence clearly does not have capacity to answer for it in law, and his carers were clearly unaware of the risk he posed.
If the decision had been taken to keep him incarcerated, this would have only led to massive additional expense for the tax-payer (surely you realise that this to be done in any way humanely, a substantial amount of additional support would need to be provided in his prison placement.)
And he would suffer more. And from the sounds of it he is not capable enough to be considered morally responsible for his crime, and therefore does not "deserve" to suffer.
Do you honestly fear he will be roaming unsupervised around zoos during his bail period? Or is this an opportunity to make a disability point.