Because the Palestinian elected government which happens to be a globally recognized terrorist organization decided to build headquarters below hospitals in hope of exploiting the rule of war you mention.

Except: doing what they did negates that rule, and all they did was put that entire hospital of their own people in the crosshairs.

This blood is 100% on the hands of the Palestinian elected government.

Because Hamas is using them as headquarters and hiding in them

Let this be a symbol of how much Hamas actually cares about the Palestinians. They prefer their own citizens to suffer than for them to be helped.