Why is the USA's nuking of Japan not considered/remembered as one of the largest acts of terrorism of all time? (>100 000 civilian casualties by modest estimates)
Because:
It was in the context of a war in which millions were dying (and in which that level of slaughter was not alien)
It was effectively, deliberately "sold" as being something that ended said war (by the people who did it, among others)
Now, one can object to the "selling" and one can object to the importance of the context. But this is essentially the reason why it was not largely seen as such a thing. Even the Japanese were more or less willing to accept it was being "justified." In the years since World War II, and especially since the end of the Cold War, there have been competing "narratives" and interpretations about the point of the bombing, their necessity, their justification, and a fuller fleshing out of how that "selling" of one narrative took place, and why, and the problems with that narrative. (I have written about several points of view here.) But if you're wondering why many people see the bombings in an essentially positive light, it is because they see it as outlined above.