I think it's because the word "genocide" has some really serious links to it (the Holocaust) so people have to be really careful using it to avoid trivializing the Holocaust (see the double genocide theory for why this is a problem)

To be clear, the use of nuclear bombs on civilians was absolutely an unspeakable horror. My concern is specifically with the use of the word "genocide" and not on whether it was an atrocity, which it clearly was. Terrorism is perhaps the more appropriate word as the commenters below discuss.