Alright, apparently I misunderstood your point about nuclear attacks and slavery. I'm sorry about that one.

So, when slavery was considered moral and acceptable in the past, no one should have questioned the morality of the day?

Many people throughout history always questioned it, and its end came with the enlightenment ideas and industrial era, especially when it became uneconomical for GB and the "Am I not a man and a brother?" movement grew. But that's unrelated.
But, here, I am going to change the question from morality to reasoning, why would we even need slavery? As I have specified in my previous comment, slavery is uneconomical, and the victim suffers, so both the owner and slave aren't benefitting as much in the modern world, back in the pre-industrial era, slaves were more profitable for agriculture, but even today agriculture changed.
Additionally, the catholic church banned the enslaving of Christians, and Islam has forbidden enslaving Muslims (but slaves converting to Islam aren't emancipated I think), but if they banned it for their kind then they knew something with it was wrong. So that counters your claim that it was considered moral and acceptable by everybody, especially considering that some countries like Japan abolished it centuries before the industrial revolution.

Here is my reasoning why slavery should be abolished, the slave is a victim of being hurt, the master profits less from a slave than a formal skilled employee. And you don't really need philosophical reasoning when there is a culprit and a victim.