Why are the bombings of Hiroshima & Nagasaki talked about as if it was an unprecedented ethical dilemma when the allies had been bombing cities for the duration of WW2?

This isn't a question about the morality nor the necessity of dropping the atomic bombs, instead I'm just curious about the narrative regarding the topic. It's often framed as if there was a huge moral debate at play and that dropping the bombs were crossing some kinda red line morally speaking. I don't really understand this framing since the allies had already firebombed cities like Tokyo and even in Europe, we had bombings like Dresden so clearly they were fine with bombing cities. Is it just the use of the atomic bomb that has caused this?