I am not a historian, but there had to be a reason. Just from the popular accounts of WW II, given the conviction of Germans to continue fighting, their ideology, self-esteem and the danger of war dragging on, obliterating a major cultural centre instead of focused strikes to military targets makes a lot of sense, both emotionally and strategically. As I see it, this was by intent not a tactical response, this was meant to be a major death blow to hit the German resolve, so they see what response will come if they continue. And to make it a lasting impact. At that point of time, the Allies knew they will win, and they felt justified bombing vulnerable non-soldiers as a means to accelerate the coming of the end of the war. Similar logic applies to destroying the towns Hiroshima and Nagasaki with nuclear bombs.