By "explain" I hope no one hears "justify" (although someone will do it anyway):

You see this with the Romans, too: they tell a city to surrender, it doesn't, the Romans conquer it and practice unspeakable brutality on whomever's left.

So what's happening? They have very limited resources, you forced us to expend them, maybe lose some of our own soldiers, and waste a valuable campaigning season - ok, now you pay the price, and let this be a warning to the next city: if we tell you to surrender, you'd better surrender.

What if the city does surrender? Are those people killed or sold into slavery anyway? I don't think historians bother recording those episodes.

It's not quite so simple, since we are looking at single examples. Alexander was predated by leaders (Cyrus the Great) who ruled in a much more progressive gentle way. Recent history has shown major powers performing very cruelly (genocides under colonialism) and there's definitely at least minor world leaders who would want to be just as cruel. So the question becomes more of why was it effective and strategic to be cruel at that scale in the past but not in the present world, and I would attribute that to technology: better communication, travel, and interconnectedness of trade.