This is a genuine question. There's probably an explanation for this I haven't thought of, but it just occurred to me and I thought I'd post it here. I'm sure a central reason would be that Japan was going to surrender, while Germany wasn't at the time.
Tokyo was also firebombed as bad as Dresden (I think about 100k civilians killed). It's also a war crime. But the atomic bomb is the atomic bomb. The US wanted to play up the terror to intimidate the USSR. The US conversely wanted to downplay the other was crimes because unlike the A-bomb, it doesn't serve a propaganda purpose.
I never really thought of the implications of the weapon used being an atomic bomb.