I would attribute it to democracy. It's very hard to sell war to an electorate unless you can persuade them you're the 'good guys'. Which is harder to sell when you're openly butchering thousands of people.
Many ancient societies had a very different conception of 'good guys' than the modern one. It was very easy to justify very aggressive war with some of these alternative conceptions.
'Good guys are the ones who worship the right god(s) and therefore we get to kill the bad people next door', or 'Good guys are guys who serve the King/Emperor and anything that advances his glory is good', 'Good guys are cultured people and everyone else is a barbarian' etc.
Democracy didn’t stop the Athenians from exterminating the Melians, every adult male being killed and the rest of the population sold into slavery, all because (according to Thucydides), “the strong do what they can and the weak suffer what they must.”
'Good guys are the ones who worship the right god(s) and therefore we get to kill the bad people next door', or 'Good guys are guys who serve the King/Emperor and anything that advances his glory is good', 'Good guys are cultured people and everyone else is a barbarian' etc.