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.
Shaun: Dropping the Bomb: Hiroshima & Nagasaki
If you haven't watched it already it is easily one of the best lectures on YT ever made.
He deals exactly with the topic of terror bombing aka "strategic bombing" in WW2. link
I think I started watching it when it came out but never returned to it. Thanks for reminding me!