The general human tendency to focus on single short term events seems to be the main cause.
Let's compare using Wikipedia as a source:
Atomic bombings in Japan:
50,000–246,000 casualties.
Air Raids in Japan:
241,000–900,000 killed,
213,000–1,300,000 wounded,
8,500,000 rendered homeless.
Mass killings of large civilian populations should not happen. I don't personally see nuclear weapons as worse than incendiary bombs or artillery. It's the number of casualties that makes it horrible.I agree. I've been to the Hiroshima Peace Park or whatever it's called in English numerous times, but I can't say I've been anymore moved by it than any other demonstration of human brutality.
I can't register a difference between a nuclear bomb and, say, a GBU-12 Paveway conventional bomb. They both destroy and kill brutally, the magnitude is irrelevant and it would be great if we never have to use either of them.
The evidence we have is
* one early bomb is more or less equivilant to one conventional HE + incendiary raid.
* 2,000+ other bombs have since been detonated, a good number of which were orders of magnitudes more destructive than the early "first gen" bombs used on Japan.
Nuclear war with the larger weapons that followed would be considerably worse than incendiary bombs, in physical destruction, in immediate deaths, and in injuries and following mortalities.