I don't think it's that. The main reason behind the brutality was precisely so that people elsewhere would find out about it, to set an example for other cities that they should surrender instead of forcing a protracted siege.

I think it comes down to economics.

Ancient cities were primarily administrative and trading centers for the surrounding territory. Most of the wealth was coming from outside of the cities, it was just collected in the cities. You could kill off most of the people in the city and still keep most of the value because you still have the infrastructure that lets you control the surrounding territory.

By the time you get to the modern era, cities are producing a lot of value of their own. If you kill off the people, you've gotten rid of the whole reason why you want to capture the city.